Most late-antique writers were surprisingly well-travelled, but it is difficult to think of a book that focuses on their travels specifically. They themselves never wrote travelogues and their journeys are generally pieced together by historians using odd references from all the available sources. The Syrian merchants you mention were for instance mentioned in a very off-hand way by the Frankish historian Gregory of Tours, not something we know about because the Syrians themselves wrote about their travels. Likewise, it is true that John Moschus, along with his friend Sophronius, was exceedingly well-travelled, but there is no book dedicated to it, since these men were important for other reasons, particularly their literary works, which provide an invaluable insight into the Byzantine life in the sixth/seventh century and are thus the focus of historical studies. If you are willing to read a rather big book on the doctrinal controversies of the seventh century, you might be interested in P. Booth's Crisis of Empire (2014), a very well-written book on the impact of made by John, Sophronius, and Sophronius' disciple Maximus the Confessor, on the Mediterranean world; it's the closest thing I can think of that ties into your interest directly. Sophronius and Maximus (a Palestinian monk) travelled tirelessly across the Mediterranean-wide empire to counter what they perceived to be a heresy promoted by Constantinople, in the process allying with rebellious governors of North Africa and Italy, as well as the pope in Rome. The first half is probably not great for you given that it is a fairly detailed analysis of the theological issues involved, but the second half is a detailed narrative of this campaign and its eventually failure; as such, it might be best for you to try to find it in a library!

You also mentioned Cosmas Indicopleustes, who is another fascinating character, but he didn't set out to write an account of his travels. Instead, his anecdotes about his travels are sprinkled throughout his (rather dull) geographical treatise, a translation of which is available here. He is very well-travelled though and one particular instance that always struck me as a moment worthy of further study is his stay in Ethiopia when the Ethiopian king was about to launch an invasion of Yemen. This account forms the basis of G. Bowersock's The Throne of Adulis: Red Sea Wars on the Eve of Islam (2013), which not only talks about the wars themselves, but also the diplomatic, economic and personal connections between individuals in Yemen, Syria and beyond. It was also written with a popular audience in mind, so I really recommend it, even if it's not a book focused on travel in particular.

Aside from these books, I can only think of a few primary sources that might be interesting. Procopius, the foremost Greek historian of the sixth century, was an eyewitness to Byzantine wars against Persia, the Vandals in North Africa, and the Ostrogoths in Italy, and thus talked quite a bit about how he travelled to various places in his Wars (translation available here). John of Ephesus, Procopius' contemporary, was another well-travelled man and his account of his journey from Egypt to Constantinople amidst the plague of 541 is particularly striking. This is translated in Pseudo-Dionysius of Tel-Mahre: Chronicle, Part III (1997) by Witakowski. Here's a brief extract from his very long description of his experiences:

At the same time that in the region of the capital these things were as yet known (only) by rumour, since they were still remote, and also before the plague (reached) Palestine, we were there. (Then) when it was at its peak we went from Palestine to Mesopotamia and then came back again when the chastisement reached there also, as well as (going) to other regions- Cilicia, Mysia, Syria, Iconium, Bithynia, Asia, Galatia and Cappadocia, through which we travelled in terror (on our way) from Syria to the capital (during) the height of the plague. Day by day we too - like everybody - knocked at the gate of the tomb. If it was evening we thought that death would come upon us in the night, and again if morning had broken, our face was turned the whole day toward the tomb. In these countries we saw desolate and groaning villages and corpses spread out on the earth, with no one to take up (and bury) them. [Followed by a long list of terrible things he saw on the road to the capital]

God's providence informed (us) about it in such a way that (news) was sent to every place in advance, and then the scourge arrived there, coming to a city or a village and falling upon it as a reaper, eagerly and or three miles (from it). And until what has been ordered against (one city) had been accomplished, (the scourge) did not pass on to enter the next. In this way it laid hold on (cities and villages) moving slowly. This is what (also) happened to this city [Constantinople]: the visitation came upon it after (the city) had been perceiving the of the visitation by hearsay from all over the place for one or two years; (only) then did it reach (the city). swiftly, as well as upon other (settlements) in its vicinity, up to one, two years; (only) then did it reach (the city).

For something a bit earlier, consider John Matthews' The Journey of Theophanes Travel, Business, and Daily Life in the Roman East (2006), which is an analysis of the journey and itinerary of Theophanes, an Egyptian official, to Antioch. For something a bit later, McCormick's The Origins of the European Economy (2001) is indeed a fantastic collection of known travellers in the early middle ages, but the section on the seventh century is incomplete and the book as a whole is more concerned with the economy. Most of the sources he referred to would be only oblique references in often untranslated texts anyway, so it might not be too useful. Still, McCormick's book is the book to read if you want to get a sense of how connected Europe was (especially in economic terms), so if you are also interested in that, don't be intimidated by its huge size!

The traveler's accounts that come to mind for me are Egeria and the Bordeaux Pilgrim. They both traveled on pilgrimage to Jerusalem and described their experiences there.

The issue is that they are written more like travel guides "here you will see the stone upon which Mary knelt..." and less like a travel journal, but you might want to check them out anyway.

The Origins of the European Economy by McCormick might be getting a little late, but he collects every example of travelers on the late antique/early medieval Mediterranean in order to make some conclusions about the economy during this period (among a ton of other things). The travelers are all collected in an appendix in the back. You might start there.