Upon reading your post I did think it could be the theme @Damo
If it's one you've downloaded then delete it and then see what happens because even though you've deactivated it the theme could still be causing conflict.
By the issue not happening in Safe Mode this does point to something downloaded onto the phone since owning the phone.
It's a process of gradually deleting items like apps etc until it stops out of Safe Mode.
You could also try > Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
As a last resort you may need to back up and Factory Reset but then when it's finished don't use your back up to reset the phone back up just incase there's something in the back up that's causing conflict.
Sometimes if a person has used Samsung Smartswitch this can occasionally caused issues.
If I can be of any further help then please don't hesitate. Take care. 😎
Daily Driver > Samsung Galaxy s²⁵ Ultra 512Gb ~ Titanium Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
