A Brittonic update? Awesome. Guess this is what my next Catholic play through will be. Will I be able to claim to be descendant to King Arthur?
No claiming descent from King Arthur. However, once you "reunify" Britain and essentially declare yourself the most powerful ruler of the non-Gaelic parts of the British Isles, you'll gain a trait called "In King Arthur's Likeness" that represents how you're sort of continuing King Arthur's legacy and how some folks might see you as another King Arthur. It can't be passed down like the Sayyid or Saoyosanht traits, but future rulers of the Kingdom of Britain can also get this trait too via rare event if they meet certain requirements (such as trait combos, high prestige, and being of one of the Briton cultures (i.e. Welsh, Cornish, Breton, Cumbrian, or Norsebriton)) representing how they too are seen to embody the legacy and/or ideals of King Arthur.
Btw, can I ask what made you guys change your heart about Cornish culture?
Before I continue, I want to remind - as you may know - that my general rule of thumb for adding a new culture (other than melting pots and courtier spawning cultures like the Jews) is that it must cover at least 5 provinces/counties, or 2 duchies, or 1 kingdom at either the 867 or 1066 starts. This "restriction", if you may, allowed me to in a sense standardize the importance and/or uniqueness of cultures relative to another on the CKII map - so, for instance, a hypothetical 2-province, 1 duchy Cretan Greek culture probably isn't that important a distinction anyways. So , in the most generous interpretation of the spread and survival of the Cornish, they would only have four provinces (Cornwall, Devon, Dorset, and Somerset) and ~1.5 duchies at the 867 start, so just a little short of the limit. However obviously this isn't a hard rule - other considerations such as historical importance, whether a decent namelist is available (or a least a good source of names online), etc., have to come into play, as mentioned below - and the fact that the Cornish could already have about 4 provinces at the 867 start was encouraging.
Anyhow, one thing Ordep pointed out (correctly) to me was that while I did not have the Cornish yet, I had the Cumbrians and the Picts, who were even more obscure and lesser known than the Cornish (but still technically met the restrictions as explained above). The Cornish were basically the only major Celtic group missing, so I guess it would be kind of awkward if they were left out.
Secondly, Ordep pointed out a loophole in my restriction. I didn't say anything on whether the "1 kingdom" meant a titular kingdom or not. So we could just make up this titular kingdom named Dumnonia, and that would make the Cornish count, right?
Thirdly, and really the most important, as you may have guessed, Ordep wanted the Cornish. Being an incapable ruler locked up in a tower, how can I can I stop my regent from his reign of tyranny?

Honestly though yes Ordep did want them in and he did a nice job with their flavor as well.