You have succinctly described the #1 hard problem in multiplayer games. There is no solution, only things you can do to do better. A good recent example was Fall Guys being basically unplayable due to cheaters and had to up their game with an anti-cheat engine to make it playable again.

The issue is that because there is no perfect solution even if you have a pretty good all server side solution you’ll still improve by adding client side anti-cheat.

Anti-cheat systems are the compromise that allow games to be played on PCs instead of only locked down dedicated hardware.