ICINGA/NAGIOS? you can even feed data already collected by Prometheus to it if you want.

ZFS or BTRF mirror will know which side is at fault due to checksums. I'm more concern about simultaneous falures of two disks. Rebuilding of a RAID puts lots of pressure on remaining disks, so probability that remaining one dies too is much higher. with RAID6 3 disks need to die to lost date, which is less likely but not impossible.

Yes, it will. Will it make any difference for you, depends of what are you doing. I would not use surveillance drive in to server, they are way too specific. Outside of that prices is pretty much same per TB/(Warranty Year) accross the board.

I done some excessive research couple of years back on the topic. you can find it here https://blog.holms.place/2022/05/01/hdd-storage-cost-comparation-may-2022.html. I do not think situation have changed match since than. Price per TB/Year is nearly constant past 8GB size.

Also consider looking to re-certified drives, or even refurbished drives. you may save hips on them. But it depends on how much you value your data, how much redundancy in you storage pool and how good your backup strategy.

Depends what are you doing. Something like keep base os patched is pretty much nil efforts. Some apps more problematic than others. Home Assistant is always a pain to upgrade and something like postfix is requires nearly 0 maintenance.

Try VyOS. I run it on APU2 myself. No GUI no convolution.

Very strange line from specs.
USB Driver Windows XP/7/8/10/11, Linux (driver free on Raspberry Pi Raspbian system)
Does it mean binary blob driver only? and you need to pay for it to use it on PC?

Stable is not "pay only" . Just build it yourself, all tools are available. it will take 30 minutes of your time if you have docker environment ready.

Nothing can beat bhyve for PFSence.

Do not try to host outbound mail on residential IP blocks, delivery will be really bad. Cheap VPS is same story. You best bet is VPS from some not well know provider, they may be avoid to be in blacklist in M$ and Google. Inbound mail is fine anywhere as so long as you can have port 25 open. DDNS works too.

Pine64 have some RISC-V boards. Not sure how far are they with support.

These IPs will be only resolved on nfsd restart. So you still need static IPs for nodes.

I was using QNAPs NASes for more than 10 years. It was a great product, not anymore. Feature bloat took its toll. It can do a lot but do it badly. So if you go for prebuild avoid QNAP. Build your own.