Since the Blues seem adamant about continuing the Buch at center experiment I think they have to duct tape Kyrou to his wing as those two definitely have chemistry. The question is who plays with Thomas? Neighbours has shown flashes of chemistry in the past, maybe try Holloway, Saad or Bolduc. I don't like Schenn there I think he is the ideal 3C. Elevating Schenn after the Sharks game was a mistake by Bannister. Yes Schenn played well but part of that was the fact that he was getting favorable matchups against the lack of depth in the Sharks forward core.
Maybe try these lineups
Neighbours - Thomas - Saad
Holloway - Buchnevich - Kyrou
Texier - Schenn - Joseph
Toropchenko - Faksa - Walker
I guess it depends on what the priority is. Which is more important, to have a more effective 1st line or a 3rd line that will have an advantage with most matchups? Both are important of course, but it's more important to give Thomas wingers that can score IMO. I agree with you that Buch and Kyrou should stick together for now, so then the question is who can finish Thomas's passes?
Hopefully Neighbours will find his scoring touch so we can keep him with Thomas....for now. The leash shouldn't be super long, though. The third member would need to be one of Saad, Schenn or possibly Bolduc (though I would prefer one of Neighbours/Bolduc on that line but not both together). Holloway didn't click there the first couple of games but he could eventually find his way back.
If Neighbours doesn't wake up soon another option could be Saad Thomas Schenn. Neighbours Holloway Texier would certainly create havoc on that third line. But hopefully Jake can stick in the top 6.
Personally I would see if Holloway can find his footing as the 3C and put Schenn with Neighbours and Thomas in the short term. Having Saad on Holloway's wing might also bring a little stability to that 3rd line. If that doesn't work, you pivot back to Schenn as 3C and try some other combinations.
I will say that I like Bannister's strategy of putting the Big 3 together at times during the game. Edmonton does that with McDrai, and it kind of gives us the option of having our best 1st line occasionally, but also a more balanced top 6 the rest of the time.