It's strange you don't have the same issues in Maps ...I've kept your suggestions in mind and I opened Maps, photos, and even in Finder, I haven't observed any green thumbnails and when I opened maps to my current location, I didn't observe any of the transparency issues either.
The only real issue I had was adding the second Gmail account to apple mail, but that may be a google issue. All current existing mail accounts that were present in Monterey before the upgrade work as expected.
Here are a few screen shots of my mac running Sonoma showing the reported areas, not showing issues for me.
The green thumbnails are for some video files (when they are fully downloaded)
No problem with gmail for me, I don't think it's linked to Sonoma .
The issues we can have are with patched or backported components: graphics and wifi drivers in or case. The rest is more or less stock.
I see you have an M395, I have an M390. Maybe a small difference here ...