Check this post for a setup that seems to work on 6430 with Nvidia NVS 5200M using nvidia-390xx-dkms including a fix for Intel screen tearing issues.
That nouveau is not quite up to the nvidia drivers I know from experience with my 2009 mbp; at times no wake up from sleep, running hot, occasional crashes when watching video etc. It’s a workaround but, at least for me, not a solution.
If your experience with nouveau is positive and you do suspect the 6.6 kernel being the issue, especially if 6.1 works, I’d simply use ‘the good one’ and stay on that since it will be supported for longer than 6.6.