Bcache can’t differentiate between data and metadata on the cache drive (it’s block level caching), so if something happens to a write-back cache device you lose data, and possibly the entire array. I wouldn’t use bcache (or zfs caching) without mirrored devices personally to ensure resiliency of the array. I don’t know if zfs is smarter - presumably is can be because it’s in control of the raw disks, I just didn’t want to deal with modules.
For your second scenario - yes you can use md under bcache with no issues. It becomes more to configure but once set up has been solid. I actually do md/raid1 - luks - bcache - btrfs layers for the SSD cache disks, where the data drives just use luks - bcache - btrfs. Keep in mind that with bcache if you lose a cache disk you can’t mount - and of course if you’re doing write-back caching then the array is also lost. With write-through caching you can force disconnect the cache disk and mount the disks.
Check with your provider for SIP server, username and password, and if they have a suggested app (even if you don’t want to use it, it means they have some kind of support). It’s probably in their support pages somewhere.
You don’t need cards to have full bandwidth, they only time it will matter is when you’re loading the models on the card. You need a motherboard with x16 slots but even x4 connections would be good enough. Running the model doesn’t need a lot of bandwidth. Remember you only load the model once then reuse it.
An x4 pcie gen 4 slot has ~7.8 GiB/s theoretical transfer rate (after overhead), a x16 has ~31.5GiB/s - so disk I/O is likely your limit even for a x4 slot.
We can’t ever stop this kind of stuff, but with something like fail2ban you can set it up to block on too many failures.
Really though - ensuring your system is kept up to date and uses strong passwords or use a SSH keys is the best defence. Blocking doesn’t prevent them from trying a few times. Moving SSH to a non standard port will stop most of the automated attacks but it won’t stop someone who is dedicated.
Not sure what you mean by this - Nabu Casa has a Zwave device already called ZWA-2 which is fully supported.