@cynerd once having build a customised kernel it is not clear to me, after perusal of https://gitlab.labs.nic.cz/turris/turris-build, how to retain the customised kernel and not pulling the kernel fed by the TOS repo feed with pkgupdate?
You are going to get core repository with packages. That one contains all kernel packages. You have to deploy it somewhere and add it as a feed to updater with higher priority than turris ones (those are priority 50).
What happens then with packages that been recompiled in the TOS repo against a new/upgraded kernel and that may not match the custom kernel revision/version?
Kernel preserves user space, that means nothing. Possibly the configuration might change for some packages and something might stop working properly so you should compile here and there new version but in general for short term testing it is completely fine to just have kernel of your own version.