Download all of your Google Photos data from Google Takeout. Then, upload said data either to something like Ente Photos with a subscription, or an immich server if you're willing to self-host.
An Immich server isn't too overly complicated to set up. You just need a big hard drive, and either an old computer that's just lying around or a cheap and small new computer. It may still take you some headaches and a period of time to properly set up, switch to and get accustomed to but the Immich documentation is pretty good and easy to follow. That plus a bit of googling should be enough of a guide to get properly set up. Beware though, self hosting is something you need take continuous care of and there may be a lot to learn. If that sounds scary I would just say go with Ente. You can still self host Ente, but you can also just pay a subscription and it would be no different or more complicated than switching from iCloud to Google Photos plus you wouldnt have to worry about actually doing the work and maintenance of running a server.