it would involve screwing Wessex to the point they can't threaten very often
Well, having southern-eastern kingdoms taking the lead could do that.
Let's assume for that some reason Wessex in Hampshire becomes a sub-kingdom or a tribal kingdom under eventual Sussex's domination...
I doubt it would mean peoples along Thames would leave Romano-Brittons alone, and Middle Anglian peoples would likely quickly deal with the divided kingdoms north-east of Dumnonia (such as Cirencester).
Still, Dumnonia itself could not only survive in one piece, but even manage to dominate places as Bath as a sub-kingdom (as for Cornwall or Glastonbury probably were).
Interestingly, it could mean an earlier Christianisation of Anglo-Saxon England, trough a more important Merovingian influence on southern England.
How would an early christianized and somehow englarged Sussex (no pun intended) would deal with a somehow safer Dumnonia and its associated kingdoms?
Of course, the Welsh kingdoms keeping some form of High King or united leadership against the Saxons would help, but that might be too much to ask for.
Well, it wouldn't be impossible to have some rulers of Dumnonia, if they get to sattelize southern-western Brythonic kingdoms, to claim High Kingship.
Effectively, it would rather mean something like Selyf's assumption of the title, as in a ruler unifying various forces against Anglo-Saxons (without real tentative of hegemony over the different kingdoms, safe regionally).
In this situation, the obvious targets would be Middle Anglians peoples, which in case of victory over Anglo-Saxons could make some kingdoms as Gwent entering into the High King of Dumnonia's sphere. How long it would last is anyone's guess, but it could mean a link between North Wales (Gwynedd and associated kingdoms) and South Wales (Dumnonia and associated kingdoms).
That said, let's be clear : we're NOT talking of any unified or even peaceful kingship there. High Kingship is an extremely unstable structure, and tendencies would be more on division than unification.
But Dumnonia could be well on its way to be considered as, if peripherical, Brythonic kingdom lasting as much as many of IOTL Welsh ones (and, maybe as a Breton kingdom from the other side of the sea as well, making Dumnonia belonging to both regional ensembles for a short time?)