I suppose you use Super+Middle Click? Not a bad idea, I dislike hot corners, and the "exposé" feature of niri is quite good. I might actually remap it to Super+Middle Click.
(I use Super+side mouse buttons to move between workspaces, I hate the keyboard-centric workflow when one hand is always on the mouse)
I don't use Super. Middle click (Button 3) = expose, Button 4 = workspace to the left, Button 5 = workspace to the right. Requires a good mouse with accessible button 4/5 of course. No key modifier.
What about software that uses those buttons? I.e. web browser? (middle click = open link in a tab, side buttons = forward/back history)
No software I use, uses these buttons for anything integral by default, neither on Linux nor Mac. And if they do, the OS has precedence. On macOS I use BetterTouchTool for those mapping, you can define exceptions for individual apps.