I don't use any "Mac cleaning tools", and I certainly don't remove system fonts.
It is a verified bug in 15.4. The issue has to do with installed custom fonts. I had validated all fonts - all were valid with no warnings or errors - and the crash kept happening. But when I deactivated all fonts I had installed, Pages launched without issue. Other users have had issues with Font Book crashing on launch since the update to 15.4 and Apple appears to be aware of the issue now.
Apple's TextKit is fairly wonky (I'm hyper aware of TextKit issues as I'm developing a word processing app on macOS), particularly as they make the move from TextKit 1 to TextKit 2. I imagine they updated either or both in 15.4 and it introduced this bug.
Edit: Should have led with thank you for responding and thanks for your attempt to help. I'm sure you encounter users who delete core components naively in this community often, but I'm well versed in the OS and am a coder, so I got a bit defensive at the word "naively" there.