You can use something like xbindkeys to map mouse buttons to key presses. Should be enough for Steam screenshots.

You could use OBS to make 30 screenshots per second and combine them into a convenient screenshot library file. :P

I've tried using screenshots taken from an OBS recording, but no matter what the quality of an OBS recording on any setting just doesn't compare to a screengrab even when set on maximum recording quality.

xbindkeys seems interesting, but I've literally no idea what I'm looking at :P

Okay, I think I have something that will suffice:
while true; do scrot -u "/home/liam/Pictures/Screenshots/`date`.png"; done
Takes a shot of the active window every second and saves it, basically what I need for taking screenshots of games in-action. Might do until I find something to easily map a mouse button :D

1. Install some programs:
sudo apt install xbindkeys xautomation

2. Learn what number your desired button has:
xev | grep state
(press keys and buttons and see output in your terminal)
Let's say it's mouse button 9

3. Create a config file:
nano ~/.xbindkeysrc
Type there:

"xte 'key Print'"
b:9

4. Launch xbindkeys:
xbindkeys
If you want to change something in that config, after that execute:
killall xbindkey
xbindkeys

im using this:

https://pastebin.com/X3gzdgxr

all binded to keyboard shortcuts.
makes a notification also so i know it succeeded or failed :)