That’s sounds very like my own situation, I have a tower that runs my main stuff, 4 x 4Tb drives for audio and video content , running JRiver Media Server. It currently serves video and a bit of music. As a retired developer , it was my main dev PC and even has SQLServer running on it , it never blinked running Roon.
The SSD for Roon is pretty much essential, the Roon Db lives on your Windows/Users/AppData . Its a read write intensive set up and Users who try with a normal HDD usually have speed issues. The Roon db is quite small <20 Gb but very speed intensive. It will be SATA not M.2 but that won’t slow things that much. I am not sure if you can get a M.2 to SATA mount BUT …
That said a 256 SATA SSD (or even a 512, is relatively cheap these days and you see the speed benefits of it all over . Startup is particularly improve.
Installation is a few screws and the SATA connector then your only issue is reloading Windows onto the new drive , Roon install is simple , “Just Another Program” installer. You could get the SSD supplier to clone your windows install drive to the new drive for you, they have the software. enclosures etc
Once Roon is running you just need to tell it where your music files live then wait. 2000 albums will take a while to load and analyze but you can let that happen in the background.
Roon imports the album then performs an analysis (audio levels etc)which catches people out they wonder why its so slow , it may take even a day or so.
BUT Definitely ethernet cable this PC to your switch and if you can ethernet to your end points. IMHO WiFi is full of frustrations. It sounds like HDMI to a soundbar is your option anyway.
Get it up and running and then repost to sort out any other issues you have.
If you want to move to a NUC later its also vey quick and easy . Just make sure you set up Automatic Backups on Roon Just in Case , also that’s key to the transition to NUC.
You may wonder why I changed , it was purely cosmetic , I was planning a new Headphone amp and wanted a small appliance liker server alongside it . The tower is a bit too big and noisy for that duty !!