In my opinion, No. Either re-purpose your existing windows comp into an unraid nas(I recommend moving the innards to a larger case like the Rosewill 4500 though) or follow the NK4 or SNAFU guildes.
Unraid is the best option, but the HP290(plex) and the NAS will still be separate devices
Basically, get a couple dense 8-14TB shuccable hard drives for it(with 2-12-14TB drives you could start with 1 12tb drive in the array, move all the data from the 6tbs, preclear and add the 6tbs to the array, then add the last 12tb as parity). You would end up with 24TB storage and 1 parity drive, but you would be able to add another parity, and 25 more hard drives.
Follow the Transcoding the JDM way with the HP290 and profit. Should be able to get 15+ transcodes out of it, and nothing really needs to be added. You install ubuntu on the 290, then add plex and link to network NFS shares from the unraid nas.
The guides look a bit ominous, but really they’re pretty straight forward. If you haven’t heard of spaceinvaderone check out his guides on unraid, some of them are a little out of date, but most are still relevant and informational. One note with SIO, Use linuxserver docker containers NOT binhex (personal preference, but once I made this switch many of my problems went away)
No, you can but you don’t need to. try it as it is first.
No
500GB, but get an ssd hard drive
On the HP290 yes, but that’s in the transcoding guide. On the NAS, you’d be running unraid.
TLDR:
Replace your NAS with Unraid Server, and replace the windows plex server with a quicksync box.