GPUs are handy for multiple monitors, local ML and AI and transcoding. I have a couple of 2070s running in a desktop for Al stuff.
Your mother board has a slot for a M2 drive. I tend to get my new machines up with a bootable USB drive, install the OS on the M2 nvme. As long as you can see the M2 drive, it is straight forward.
My little pi does not always see the drive, or the drive adapter.
500 W PSU might be a bit light if you plan on adding a graphics card.
For Plex and arts, you have more than enough power. Some arr mavens recommend an SSD while downloading and unpacking media files, followed by atomic move to a HDD.
I am trying to get a M2 SSD on my pi server to see if media file searches and the Plex interface run a little bit smoother. It is hard to find a adapter/m2-ssd combo that I can afford and does not cause the Pi to vomit.
I run the pi OS and support functions for the apps off of a M2 SSD that is attached via one of the USB 3.0 ports. The HDDs for media are off the other 3.0 USB in a low end NAS. I have 2 and 4 bay units. The media server has a 2 Bay drive with 2 X 10 GB drives. I have about 14 or 15 GBs of movies, TV, and music. As people get new movies, my brother rips them and I distribute to the setups at everyone’s house. Yeah we kinda have the same movies.
The 4 bay setup is a raid 10 for my photos, and other files that I can’t afford to lose. I run paperless ngx and pydio. I am swapping out pydio for photo prism. I have 20K plus photos (33 MP images). Raid 10 is complete redundancy, hot swapsble. I scanned a number of old family photo albums. Kind of a single, safe repository.