u/Zooasaurus has already provided an exquisite answer to this question. I thought I could add just a few interesting details about Liman von Sanders' time in the Empire. Before getting started, I would like to thank u/Gankom for bringing your question to my attention.

Liman von Sanders is almost a traitor in Turkish popular imagination. This is rather odd considering the man spent considerable time and effort in fighting for the Ottoman Empire in the Dardanelles Campaign and elsewhere. During the Dardanelles, it was none other than him who (at least de jure) led the Ottoman Army. The Allied forces prepared to land on a strategic location on 25th April 1915. A younger Turkish officer decided on his own to move the 57th Infantry Regiment to a position from where they could counter-attack the Allied landing and prevent them from cutting the connection between Gallipoli and Istanbul. The counter-attack proved vital and decisive as the Allied forces' advancement was thwarted. The commander who ordered the counter-attack was Lt. Col. Mustafa Kemal (later Atatürk). Here was the story of a German commander who failed to either anticipate the Allied landing at proper time and location, or worse, who (for some reason) turned a blind eye to him. That Liman von Sanders would have deliberately turned a blind eye to such a crucial movement does not make much sense. Being a German officer, elsewhere he could find it in his country's interest to prolong the campaign, but a successful landing could effectively take the Ottoman capital out of the war and this would hardly serve German or Ottoman interests. Be it as it may, still, Liman von Sanders is the symbol of the old, complacent or ill-meaning German officers whose misdeeds were corrected by much Ottoman blood and by the astuteness of the likes of Mustafa Kemal. It does remain true that Mustafa Kemal took the initiative by himself to order the counter-attack; von Sanders confirms this much in his memoirs (p. 112). But this surely does not mean the German was deliberately detrimental to the Ottoman war effort as he (like any other commander at any time, even today) was not omniscient.

Granted, the Ottoman officers at the time most likely did not hold such strongly negative feelings towards him. But, to respond to your question about his impact, it seems indubitable that he also had some negative impact on a personal level. Enver Pasha, the young minister of War, seems to have found himself at odds with the German at times. Indeed, in his memoirs, von Sanders describes occasions where Enver (who, by the way, is thought to be a relentless Germanophile in the popular imagination) actively sought to limit his authority (pp. 19-20). As u/Zooasaurus has already pointed out, there was not all that much work for him to do and at times the Ottoman officers seem to have tried their best to limit his influence and impact. He did not reform the Ottoman Army per se; his role seems more to be that of presiding over it and making sure it functioned well and, presumably, in line with German interests.

If you are interested in him, I would recommend you read his memoirs, available in Turkish, English and German. It provides us with the man's own perspective about the events of the Great War and at times it is quite frankly, well, funny. At one place, he likens Cemal Pasha, the Minister of the Navy, to Garibaldi (the beard is unmistakably similar but the similarity never occurred to me before reading the memoirs!) and says he seemed to be a man who always kept his true opinions to himself (p. 14). At any rate, the details he provides about his cooperation and confrontations with the Ottoman high command, in particular with Enver Pasha and other Unionists whose conduct he generally disliked (p. 73), is interesting to read.

The source is the memoirs of Liman von Sanders. I have used the Turkish version here titled Türkiye'de Bes Yıl, ed by Nurer Uğurlu et al, trans by Örgün Uğurlu (Ankara: Cumhuriyet, 1999).

I’ll try to answer your question, sorry if it doesn’t satisfy you

All in all, contrary to popular belief the German military mission under Liman von Sanders doesn’t have a big impact on the Ottoman army. There are several reasons why

First, the mission arrived rather late, on 14 December 1913, just mere nine months before the Ottoman Empire entered the war. Their arrival caused a diplomatic crisis with the Russians, which prevented them from doing any work for two months. This was further compounded by the shuffling of posts - many are even before they had assumed their initial posts - which prevented them from effectively affecting the branches they were assigned in. For example, Kress von Kressenstein spent less than three months as the director of the artillery branch school and training center before being reassigned as the operational branch chief of the Ottoman General Staff.

Secondly, members of the military mission aren’t high caliber officers fit for this kind of mission. Liman von Sanders himself wasn’t a particularly bright or intelligent officer, who only achieved his position of divisional commander by sheer diligence and loyalty. With some exceptions, most of them were mediocre officers who lacked the necessary background or talent to perform their highly demanding jobs. Most of the German officers had never served overseas and the resulting adjustment process to a new country and army, combined with total lack of language skills and cultural awareness of local conditions, means that the officers were unable to fully acclimatize with their new environments and were unable to affect the Ottoman military at the outbreak of the war. Consequently, there was considerable resistance to the advice that members of the mission gave. Liman von Sanders himself stated that he had a difficult time in making his weight felt in Ottoman military affairs and was frequently discouraged from doing so. Additionally, the German officers were heavily scattered throughout the empire to the point that there were no Germans assigned to the newly established Ottoman Army training center.

Thirdly, the German Military Mission did not make any institutional or structural changes in the Ottoman army. The general unit architecture, the innovative triangular division concept created by Ahmed Izzet Pasha in 1911 remained the same. The reserve and recruitment system that has been immensely improved after the Balkan Wars remained the same, and there were already been massive transformation and reorganization of the entire field army headquarters system, officer education and training system even before the arrival of the German mission. Additionally, the modern German military system was already well established within the Ottoman General Staff officer corps, hundreds of its members already passed through training in Germany beginning in 1885. So most of the members of the military mission merely helped to continue the ongoing process by providing guidance and additional influence for these German-trained officers. I believe the only major change that the mission does at this time is the reorganization of the Ottoman General Staff by Bronsart von Schellendorf to mirror more of its German one, with him elevating many talented Ottoman German-trained officers as branch chiefs.

One reason of the capabilities of the Ottoman military in the First World War, rather than the German military mission, were because of the various reforms and discourse by the members of Ottoman military after the 1908 revolution and the defeat in the Balkan Wars, in what Mesut Uyar called an “Ottoman Military Renaissance”. With the restoration of the constitution in 1908 and the erasure of censorship, officers began to write and publish their thoughts and ideas about various aspects of the military, especially their combat experiences and criticisms of past campaigns in various military journals and literary clubs. This was further intensified with the Ottoman defeat in the Balkan Wars, which further encouraged officers to openly voice their opinions and ideas for the what was seen as a weak military. Some of the most favored topics were the performance of the Ottoman General Staff, the outdated and cumbersome methods and procedures, the mechanism and dynamics of attack and defense, new weapons (especially machine guns), combat support and service support, and the recruitment and training system. The writings of these officers were taken seriously by the military authorities, and were used as basis for various military reforms and improvements like the 1909 officer reform, recruitment reform, and the creation of the triangular division.