Since we're talking about VDI and thin clients...
I just now got around to hooking up a Raspberry Pi 4 to a 4K monitor. I've had it since July but only been running it headless.
Now I'm running RDP on Remmina and it actually pretty decent. It also works as a semi-fat client running light stuff such as office, watching youtube, news sites etc. Not as fast as an i3 or something with more horsepower but reasonably good.
Right now I have two Firefox windows with a couple of tabs open in each and running a RDP session with 2560x1440 resolution and have a couple of terminal windows open as well, a gimp file and looking at HD video with VLC over the network. Using about 1.5GB of RAM now. I'm running Raspian OS, which is Debian 10 with some tweaks.
It's the 4GB RAM version I have. But looking at the memory usage I think the 2GB RAM version would do fine in most cases as well.
2GB RAM version is $55 if you buy the bulk version. Add a suitable case $10, power supply $8, 8GB memory card $10.
Total cost around $85.