This is just a couple of ideas, a 500gb ssd isnt very much and can really help with file transfers. I've hurt an ssd doing this after a year or two (it wasn't a very good drive). The second is that transcoding causes a bunch of read and writes to either the cache drive or the spinning disks depending on the setup causing un-needed wear.

This made a huge difference on my system and has a couple of benefits. The second thing you can do is to have plex transcode to RAM. This way you are not bottlenecked by the spinning disk.

This will help improve the responsiveness from plex because it will have substantially quicker access to the databases/images. Something that may help and is defiantly worth trying is to add a cache drive if you dont already have one and lock your plex docker to use it. I have a 500gb Samsung SSD for my unraid machine with 24Tb of Storage and it has been more then adequate for plex, file transfers, a couple of minecraft servers, and home assistant running in a Vm. The cache drive can greatly benefit performance though just no in the way you are hoping. I doubt that the mechanical drives are your issue and adding the 2tb ssd wont fix the issue.

Appletv 4k plex