That sounds great!
Hmm I was intending that the Brythônys would view Bretwalda as a separate title, given by the English, with Imerâdôr/Âlt Rî being the one they gave Arthur several years later, but we can do both(sorta). They used to be separate titles in official documents, but by the 8th century they became the same title, just in different languages. I like your idea about how the Coelings view the title btw!
Thing is, there can't be an "Overking of the English" as you intend in TTL. The Germanic tribes are too disparate and there settlement wasn't really exclusive ethnically by intent (even if that was the result). OTL Bretwalda was used to refer to the overking of the lands south of the Humber and then became synonymous with "overking of Britain" which included the Welsh kingdoms. ITTL the Angle and Saxon tribes will see Arthur as the Overking of Britain and name him accordingly, thus will be synonymous with his claim as sole
Dux Britanniarum, Comes Britanniarum, et Comes Littoris Saxonici per Britanniam. Now the 3rd title is something that can be worked with if we mutate to mean command over the incoming tribes:
Comes Littorum Anglici et Saxonici, Count of the Angle and Saxon Shores, but Arthur's High Kingship is based on ruling all Britannia and the Angle and Saxon Kings won't see themselves as separate from that; your TL precludes them being strong enough to separate themselves and provide a High King over just themselves!
I thought the actual POD was far before Charlemagne's rise to power? That said, I can still see the Karlings becoming major players albeit perhaps not doing quite as well and perhaps not successfully invading pagan Saxony.
Rather than primogeniture, might there be a narrowed version of Tanistry? As in: letting the noble families have their say on who should be the Âlt Rî, but limiting the options to the sons and grandsons of previous kings, or somesuch?
Primogeniture is overused and inevitably leads to incompetent kings. Tanistry might be more of an Irish thing, but adopting a similar idea should not be unthinkable for other Britons.
The Germanic kingships evolved a similar custom to tanistry as the role was elective in origin; the rise in noble lineages meant that kings became elected/selected/confirmed from among the previous king's kin, often in line with his will/wishes, and meant that the best candidate was usually the eldest son. Compare the early AS kings of Wessex and England with the Holy Roman Emperors and early Kings of Denmark.
