I wanted to press the side button on my mouse and go to the game's pause menu so I could cut the characters' lines faster

I don't speedrun this game, but I'm not sure if this is allowed.

After reading the game's rules, it says:

Remapping keys without explicit permission is not allowed

The use of more than one keyboard or mouse is not allowed (except for the use of a numpad)

The use of an external device that has keys that allow the rebinding of inputs is not allowed

The use of an external device that has keyboard inputs on it is not allowed

Note: These rules include, but are not limited to, the sayodevice, wooting uwu, bluetooth keyboards, multiple mice, multiple keyboards, stream deck

And in another area:

Rebinding keys (outside of the escape macro) is not allowed

No macros are allowed on the scroll wheel

What you're referring to is the Double Esc Macro. It is perfectly allowed to rebind this macro whatever key or mouse button you like, however the Double Esc Macro no longer works on patch 0.93 as the devs have patched it. To skip dialogue now on patch 0.93, you need to open the ingame console and type "skipdialogue" and press space to skip dialogue. Creating a macro for space however, is not allowed. Go to the guides tab and watch the beginner's guide to MiSide speedrunning for further clarification & instructions.