I have Openhab 2.5 (Upgraded 2.4) running on RPi 3B with Stretch.
I think upgrading to Buster with fresh 2.5 (Don’t know why actually, everything works :))
but I need advice what method I should take:
I think if your system is running fine, stay with Stretch. It is still supported for a while. Otherwise option 2 makes sense.
I personally would shy away from running Docker because the Pi has very limited RAM anyway.
It sounds good to me.
I am lazy and have just copied configuration portions between systems. perhaps taking the opportunity to delete unused portions and simplify others.