As long as you have more hard drives & sata ports available; yes. Copy data to temp destination; destroy, rebuild, copy back.
Otherwise? No.
By destroying the pool 
I’d just backup whatever app settings manually, and if the app has a backup or a config file, I’d try to find it & save a copy of those as well.
You can, but it would bring things over. Honestly when I clean install & want to keep ACL, I straight up go & screenshot everything, so at least I can mindlessly recreate instead of thinking about it.
If you have a free sata slot, you could mirror the nvme boot drive & the sata boot drive. Then I guess you may have to CLI a bit & zpool detach the now missing nvme… honestly since you’re half considering a clean install, it may just be easier to do that.
It doubtfully is helping. L2ARC, SLOG, and svdev all have their place - ~gross oversimplifications ahead~ but that place very likely shouldn’t be on your home server. They are either adding nothing of value or degrading your performance. Maxing out RAM is likely fastest performance gain.
It’s all good man - many first timers falls for the ‘more hardware = more betterer’ trap.
Good reading: When to use a NVMe cache - #5 by Protopia
I sadly don’t use Immich, so can’t advise - but many apps have the option to backup configs; otherwise, you’ll have to find where those files are & save them in advance.
I’m also hoping you’re going to be using the nvme to host the apps/vms as part of the rebuild. Maybe good idea to add a 2nd one for a mirror?