Along these lines, it would be neat to use Raspberry Pi or something to make a "terminal to HDMI adapter", allowing me to have an unlimited number (tens or even hundreds) of external monitors for my laptop, each running a separate terminal-mode program such as a text editor, htop, Bloomberg, etc.
I often find fully working LCD monitors discarded by people who have no use for them, and could easily collect five or more of them that way if I had a convenient way to "wire them up" to my laptop.
You kind of can do this already with a raspberry pi strapped to the back of a monitor and "barrier", the successor to synergy, to use the keyboard and pointing device of your laptop on as many displays on the same desk as you want. Each one is an independent computer.
https://github.com/debauchee/barrier
Unfortunately the new price of raspberry pi is kind of ridiculous.
If you have enough USB ports, you should be able to get away with using DisplayLink dongles for the video part. DisplayLink doesn't require any additional video processing hardware (unlike DisplayPort) or even USB-C ports, you can run the old DisplayLink standard over plain old USB 2! Not great for playing video, but a terminal app should run just fine.
I think the current driver restricts you to 4 displays but I'm not entirely sure why (you can do 6 on Windows so I think it must be a driver issue).
Of course, with your RPi idea, you could connect one computer to dozens or maybe hundreds of serial ports, but if all you want is five more screens, USB should do fine for your needs.