Deskmat under the keyboard

Tape and/or foam mods in the case (will change the pitch of the sound, which might disturb less)

O-rings, but these will change how they keyboard feels. I didn’t like them

Foam inserts for keycaps - like o-rings but foam. never tried them

Lubricating the stabilizers - should mitigate some space and enter key rattle

If your keyboard supports hotswap, new switches are a more expensive option.

Edit: The person who invented markdown WYSIWYG text editors is on my shit list

They will absolutely try to force the paid feature on you. How many times have you seen the youtube premium prompt?