@FreePietje
I actually tried in this order:
1. PARTUUID, resulted in kernel panic;
2. UUID, after reading somewhere this was better method, still resulted in kernel panic;
3. Reverted everything, so back to "root=LABEL=RASPIROOT", which strangely still resulted in kernel panic on boot. Maybe all caused by the first attempt breaking something.

I need to start a fresh attempt using LABEL method.