Not sure of the reason why, but the BrianApp's resizing utility Sizer 4.0 causes massive problems in Sea of Thieves, even when not actively in use.
It only affects mouse movement, using a controller results in no stuttering.
Closing Sizer out of the system tray immediately causes the stuttering to cease.
Changing your mouse hz can lessen the problem, but you don't even realize how bad it is until it's gone completely.
Know this will be useless to most of you, but Sizer is a relatively popular utility for resizing windows, and a fair few content creators use it to capture pixel perfect footage.
Unsure if related to EAC.
Just figured this knowledge should be published somewhere searchable.
Older footage showcasing how bad it was at high Hz mouse rates: https://www.youtube.com/watch?v=27uebqG1K5o