Devices dropping and being added with a new ID won't be automatically re-added, you need to reboot then run a scrub, also see here for better pool monitoring.
Please set the DHCP leasing time to a much larger value, like a day or so, or it keeps spamming the log every 5 minutes making it much harder to look for other issues:
Aug 21 03:05:16 Unraid dhcpcd[2099]: eth1: leased 10.0.10.112 for 300 seconds
Cache filesystem is fully allocated, which shouldn't really happen on recent kernels, but you need to run a balance, see here for more info:
https://forums.unraid.net/topic/62230-out-of-space-errors-on-cache-drive/?do=findComment&comment=610551
You should also update to latest beta.
Except WD SMR drives before they are written to, since the firmware knows there's nothing on those sectors in the platters they return the data from the cache, making them look much faster then they actually are.
It's unusual, but difficult to guess why, could be due to unclean shutdown(s), hardware issue or just a filesystem issue, if it happens again to other disks likely there's some hardware issues, but most likely it was an isolated incident.
You can get an 8 port LSI like the 9211-8i or 9300-8i and a SAS expander, get multiple 8 port HBAs, or get 24 port HBA, but those are usually more expensive than a HBA plus expander.
Difficult (if not impossible) to find NVMe + one SATA in the same controller, there are dual NVMe + SATA m.2 adapters, but the SATA m.2 requires an available SATA port on the board or other controller.
Also have this one, same chipset, it supports 5 SATA devices total (not NVMe), m.2 or regular SATA ports using the bundled adapters:
https://www.amazon.it/PCI-Adapter-Card-JMB585-chipset/dp/B087613YTK/ref=sr_1_7?__mk_it_IT=ÅMÅŽÕÑ&dchild=1&keywords=jmb585&qid=1597944207&sr=8-7