OH MAN I FOUND HOW TO DO IT!
I was seeing the same thing you were and didn’t see any alternatives, but at the same time apple and google seem to think it’s not a problem 💀
So I was googling progressively weirder phrases about transferring between Google Photos and icloud. Idk what exactly I wrote but this time the AI mentioned an unintentionally “secret link” that will show icloud as the only option for transferring from Google. AI was quoting from a Reddit post and I tried clicking basically every link in the comments looking for it, and then suddenly I found it 🤩
The link takes you to an Apple page that is about transferring from Google and it provides a link embedded in the text. When you click it, you sign in to your Apple and Google accounts etc and then the screen with drop downs of who your transferring to contains only icloud and nothing else! I’m losing my mind why would this link be so buried like this 😩 I can’t even find the original Reddit post in my history on my Reddit app or in safari so honestly maybe it’s haunted?
anyway yeah my photos are transferring over, got a confirmation email and everything.
Here’s the impossible to find Apple Support page. Transfer a copy of your photos and videos to iCloud Photos - Apple Support Click the extremely normal looking Google Takeout link:

Below is the stupidly hard to find link. For TL;DR —> click the link below. It’s exactly like the other Google Takeout page except it lists only icloud as an available destination to transfer to. It’s a separate Google takeout just for Apple icloud: https://takeout.google.com/takeout/transfer/custom/photos
[Edited by Moderator]