Try no_root_squash , your current setting maps root on client to nobody on NAS.
Either way, as of today (this thread is 2yrs old), I would go for a different solution now that ZRAM has become available in openHABian.
I also used to run my OH on a box to mount write-intensive stuff of my NAS but there’s drawbacks to this such as a dependency of availability on more external systems (NAS, network). These can be particularly nasty e.g. on power outages.