I wonder if you dropped down to Win7. I'd also try and de-bloat the OS as much as possible.
I'd say it's likely the 8GB of RAM. Win10 and above really start to lean on system RAM, and 16GB becomes minimum. For your purpose though, a de-bloater app should be fine. Just make sure it's not a sketchy app that's going to put some keylogger on your rigs!
It's Win10 being a bit of performance goblin with more recent versions. An SSD's really smooths these rigs out. I've done that upgrade tons of times to get more life out of them.
Basically it comes down to Win10 performing terribly on a HDD by default. It tries to do to many things at once and just runs outta bandwidth. Slimming down Win10 was a good idea.
SSD will go further than the ram. So I would do that upgrade first and see if you want to try and snag more ram. Upside is DDR3 is super cheap and easy to find used right now. So upgrading further should at least be cost effective
Also don't forget that you can use various types of cloud gaming services. Including the ability to do local streaming via AMD Link.
p.s. They also run Win7 fine. My work environment had them on 7 till the bitter end of the support window for Windows 7 . But then you have to isolate them off from the regular net, as Win7 machines are easy targets now that they are outside of getting regular updates & support.
Some of the later generation versions of these towers came with MSATA to 2.5" SATA adapters. Which I always thought was a funny thing to do instead of just going with regular 2.5" SATA SSD drives. I guess for Dell it made sense as they could just use the same MSATA drives all over the place (desktop & laptop, older and newer)
Not sure that would work with a PCI-E M.2 SSD drive though.