the single best thing you can do is fit a sata 1 or 2 tb SSD in place of the spinner. Its probably not worth pulling the mobo to fit more ram, or looking for a pci ssd slot. trust me, the sata ssd alone will transform your computer. I use crucial ssds in 1/2/4tb sizes....
As a coincidence, last week I bought a used late 2015 retina 21.5" iMac second hand very very cheaply. I could not believe how slow it was when I got it home (I have a couple macbooks and a mac mini from that period that run circles around it). Just checking system preferences was a spinning beach ball!
Then I was super disappointed to find out that the RAM is soldered to the logic board and not upgradeable and the only drive port is SATA! I decided to upgrade it anyway, removed the screen and put in a cheap 512gb SATA SSD.
It is a totally different computer! Not a powerhouse by any means but you can actually use it without feeling like it is a chore.
It's shocking that Apple released that thing with a 5400rpm HDD. Honestly, I can understand trying to maintain a profit margin but where is the pride? Increase the price but don't release that garbage into the wild! (In fairness, maybe it wasn't so obviously terrible with the OS it had at the time but Monterey is brutal!)
I'm curious about the comments that external TB3 would actually be faster than an internal SATA drive - from what I can see, this device just has USB3 and TB2? Apologies, I've never explored using an external drive for the OS before.