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.
Based on what I’ve seen with my use of ZRam I don’t think it reserves the total space, but instead consumes whatever is shown in the output of zramctl --output-all. If you’re swapping then yes it would take memory from the system (up to the 8G disk size), based on how compressible the swapped content is (like if you’re getting a 3x ratio it’s 8GB/3=2.6GB). That said - it will take memory from the disk cache if you’re swapping.
Realistically I think your issue is IO and there’s not much you can do with if your disk cache is being flushed. Switching to zswap might help as it should spill more into disk if you’re under memory pressure.
YouTube blocks it. There are extensions to allow it (like Vinegar) but by default it’s blocked. Brave might work around YouTube’s block in the same way.
Not sure what you mean by this - Nabu Casa has a Zwave device already called ZWA-2 which is fully supported.