Hey thank you for your reply!
The skylight hadn’t even crossed my mind. I was using “Captured scene” as the setting for it, it was also set to be static. I’ve now been playing around with a few of the settings on the skylight and a bunch of the general lightmass settings and I think it’s improved a bunch already! The issue is still there but way less noticeable at least. The problem is reduced a lot when I make sure to keep overlapping faces to a minimum too, it’s just that some of what I’m doing would require a crazy amount of splitting to make sure there are no overlapping surfaces so that just wouldn’t be worth it at this point I think.
Anyhow thank you for your reply, it helped me learn a bit more about the functionality of the skylight too which is great!