Pre-Proto-Celtic

???

Late PIE made tk become kθ.

I don't think you understand how language formation really works. In order for there to be Brythonic languages that share the same linguistic developments, there has to have been a single proto-language, which is called (Proto-)Brythonic or Brittonic.

I don't think you know how languages work. One word: sprachbund. Also, Proto languages are conlangs. Not only do they ignore dating, they rely on sound changes within an entire island with eight dialects, a literal sea separating one of them from the rest, even in isolated populations. It's laughable. Absolutely comical. Cornish and Welsh were already distinguishable in the 3rd century AD (retarded development of a: > o in Cumbric, lack of mb > mm in Cumbric, Rhys, Koch, even Jackson, etc etc), before that wiktionary Conlang could even hope to exist. That construction is laughably awful. It was never spoken (see above), is severely, severly misdated. They’ve kept ɣ which disappeared in all reflexes, have totally ignored the fact all British languages kept a case system into the 8th century, and don’t know how conjugation works, using MODERN WELSH’s irregular conjugation. For 1500 year words unrelated to Welsh conjugation. Also applying the wrong sound changes in the process, like i-affection, which they have colour letters that had already been syncopated a century before. It’s literally a farce.

Also, I think you're confusing the pre-Proto-Celtic thorn cluster with the later Brythonic development of -rt- to -rθ-. The fact that all other Celtic branches show a -t- means the thorn cluster had already resolved itself to -xt- anyway.

Yes, I'm confusing sound changes separated by 3000 years. Very likely indeed. Proto-Celtic already had spirantisation after r in the first-century. I won't repeat my sources. Like Arθos.