I did this recently with a 170GB Google Photo library. I wanted to move from Google to iCloud photos. What I did? Use Google Takeout to download all the photos to my hard drive. Then I used my MBP to import all the photos into a Photos database. I used a photo duplication app to wipe out all the duplicates. Google, unfortunately, has MANY duplicates because of the auto-enhance feature and shared photos.
Anyway, then I turned on iCloud Photos on the MacBook. This uploaded to iCloud - it takes forever, don't worry about the tally at the bottom, let iCloud do its thing. It takes a LONG time (took me a week). But once it gets up, everything is good. I then downloaded it back down to my iPhone (Xs MAX 512GB) - spend a lot of time plugged in.
Points: Let it do its thing. Don't restart it, don't try to get it moving, it'll move when its ready after doing all the file creation, etc... Give it a few days to calm down after upload.
Yes, you'll have to re-do your albums. But I like iCloud Photos a lot, it works really well between all of my iOS devices and Mac OS device. I like having the ORIGINAL file on my MacBook Pro that can be backed up vs in Google's cloud. It works really well. I can save a photo on my MBP and it instantly be available on my iPad and my iPhone, and vis versa.
Check this out for a longer version of my experience:
https://forums.macrumors.com/threads/photos-library-too-large.2114848/page-3#post-27032882