Recently i started looking into seljuq history and what made me curious was how it only lasted 150 years despite being so powerful, at first i thought it was powerful outside threats that brought it down, but that cant be as the mongols only struck the final blow, and the crusaders were more of a nuisance. Then i thought it was the instability, fictionalisation, feud between the caliph and sultan, infighting, a flawed political system and disloyal atabegs and amirs. While this is most likely the case, how did the HRE, which basically had all the issues noted above last for a millenium? Also side note is that i would like to ask if their are any good reads/ sources on the seljuq civil war during the 1100s, i see it as an immensly underrated and relatively unlnown period that has little sources written about it, best i can find is this https://digitalcommons.ursinus.edu/cgi/viewcontent.cgi?article=1010&context=history_sum
My interest in the Seljuqas is more of a side-passion, and I'm also not an expert on the political history of the HRE, but I would suggest that the premise of your question here is a bit flawed. Only by the very loosest possible definition did the HRE last for a millenium - the discontinuities between the empire of the Carolingians, that of the Ottonians, the Hohenstaufens, and the HRE of the early modern and modern periods are so great that it really can't be seen as one state lasting for a thousand years in any meaningful sense. A more accurate comparison would be the idea of the HRE to the concept of the Islamic caliphate - a persistent political and ideological idea, yes, but not a consistent geopolitical entity. The 'Abbasid caliph is still there during the Seljuq period, and still has an ideological role - but the real political power is in the hands of the regional Seljuq dynasties, much like the dispersed political power of the HRE.
Comparative history like this is, if one is not careful, a tool that can obscure as much as it can illuminate. Perhaps there are insights to be gained from a comparative study of imperial-scale political philosophy here, but I would suggest that even then many of the basic underlying elements are so dissimilar (I'm thinking geographic and climactic, technological, cultural) as to render any more detailed efforts at comparing the particular dynamics of administrative systems, regional politics etc fairly unproductive.
I'm not hugely familiar with the specialist historiography, but the only real starting point for Seljuq history is Andrew Peacock's The Great Seljuq Empire - it's the only major overview in English. Research into the Seljuqs suffers from the general problem that it requires enormously broad linguistic skills - at a bare minimum, Arabic and Persian for the primary sources, and English, Turkish, and probably both French and German as well for the secondary literature. There aren't many historians who can claim mastery of all of these! Many of the contemporary authors deliberately wrote their works in an almost hopelessly impenetrable, convoluted, highly literary style as well (and anyone who's ever tried to read some of the scholarship produced by German Orientalists in the 20th century would be forgiven for thinking that they'd decided to imitate the sources they were studying!).
As for other reading: I'd also reccomend C. E. Bosworth's The Ghaznavids - not about the Seljuqs but rather another Persianised Turkic dynasty which preceded them in what is now Afghanisatan. There was also an exhibition of Seljuq art and artefacts at the Metropolitan Museum in New York a few years ago; the exhibition catalogue is called Court and Cosmos: The Great Age of the Seljuqs and can probably be found floating around on ABEbooks or similar - well worth a read and plenty of glossy pictures!