Is there any way to monitor all functions of a certain address with something like contract.on(event)?

Sentinel by openzeppelin defender is one of those o/

If you wanna do it raw, i think you should rely on mempool or by reading each new block

yeah but i want to do it on bsc and i don't think the code will be fast enough to catch every tx, about the mempool sometimes i need to read private transactions (through relays) that's why this doesn't work

Give it a try then
Defender.openzeppelin.com

I'm not using it yet on production but it's a really really powerful tool