Sure, but I am just talking about the expected influence of Brythonic on Old English, which is an unsurprisingly low amount

Me too honestly, I’d love to be able to know for certain but unfortunately a high level of certainty is very unlikely to ever surface.

But who knows, each subsequent study on the Anglo-Saxon migrations have shed more and more light and offered more and more concrete and well-evidenced hypotheses on this era

Although I do agree with you, that study claimed a continental North Sea ancestry was as high as 76% in the skeletons they analysed.

The 50-100% example is an example of an older and relatively lower quality study that can’t be used as well to analyse the DNA of groups from so long ago, as the paragraph just below it says:
>”However, populations change over time through drift and gene flow, so present-day populations may be poor proxies for ancient groups of unknown genetic makeup”.

Honestly English having very little Celtic influence is to be expected and isn’t considered all that unusual, at least not by historians such as N. Higham and J. Davies.

This video goes into more detail on the subject but in short, elite, “high-prestige” and (typically) conquering languages generally do not borrow all that much from the low-level conquered ones. There are exceptions of course, such as French and Gaulish, but for the vast majority of conquering languages it does seem to be the case.

The theory that there was NO Anglo-Saxon migration is extremely fringe, and doesn’t appear to have any sort of wide level of support.

I answered a question on AskHistorians earlier today about why the Anglo-Saxon migrations are so controversial, and what the various theories have been over the decades, which might be useful to you (although I am by no means an expert) https://www.reddit.com/r/AskHistorians/comments/xwkhb8/can_someone_why_new_evidence_of_a_large_scale/ir9cskt/?utm_source=share&utm_medium=ios_app&utm_name=iossmf&context=3

To summarise, the most modern study of Anglo-Saxon remains concluded that there was a large-scale migration from the continental North Sea, however unlike previous theories (and the very dubious primary sources of Gildas and Bede) it doesn’t posit that this took place as a singular event, but rather that it took place from sometime during Roman rule in Britain to as late as the 8th century.

In addition, this video goes into detail over why the evolution of English is not all that unusual. In short, most elite-level (or “high prestige”) languages do not borrow heavily from the lower languages. There are exceptions of course, such as French having quite a lot of Gaulish influence, but the vast majority of the time the conquering language takes very little from the conquered.