Hi there,
This is very unlikely a theme issue so not sure what we can do to help here.
If it’s a theme issue, it would happen as soon as you install GP and not after 3 weeks of smooth sailing.
Error 500 is generally caused by the server so I’m WPX would need to look into this for you.
2.7 gig is definitely very large. The GeneratePress theme is 935KB and GP Premium is 853KB.
I would recommend going through some debugging steps like disabling all plugins etc to start.