You can using ODIN as long as the option to block a downgrade hasn't been used. It wasn't for the initial Android 10 rollout. Just be aware that you might get a few bugs when you do: the adaptive brightness & Samsung health functions might be buggy. It may also wipe your whole system. This is what happened when I downgraded from 10 to 9 on the Galaxy A70.
No you're not. I'm saying it's possible but there are risks. If you want to I'd suggest going to the official Samsung service centre. They can do it for you. They've got the system images.