The FS feature is great, it’s just cumbersome to use without a tool.
Snapper works well for a local backup like history both against botched updates and accidental deletion, but eats up the free space with the default settings.
Timeshift is an easy to use GUI but doesn’t support non-default partitions.
Also the quota support had a nasty side effect: freezing the whole system on snapshot deletion.
I think calling it a “cache” is not precise. The primary function of the DRAM is to hold the dictionary for translating logical addresses (e.g. sectors) from the OS to the physical addresses (which NAND chip, which bank etc.). This indirection is needed for the controller to do wear leveling without corrupting the filesystem.
On a SATA SSD without DRAM each read IO could mean 2 actual reads: first the dictionary to find the data and than the actual data being read. As you said HBM helps by eliminating this extra read.
The read and write caching is just a use of the remaining DRAM capacity. Since modern Operating Systems use the general RAM for the same function it is usually just a small increase to the throughput.
There is an even more relevant video of using external storage trough USB. He recommends using software raid:
It is not necessary but still better to run both online discard and batched fstrim. from man:
Also, a device may ignore the TRIM command if the range is too small, so running a batch discard has a greater probability of actually discarding the blocks.
The mobile and TV clients are often limited to the codecs with hardware acceleration. Or just selecting a lower bitrate on the client will cause transcoding.