Moving those two domains to iCloud+ is possible and fairly easy - If you follow the steps in the configuration on icloud.com in "Custom Domains" you'll start receiving emails into your iCloud for those domains / email addresses.
However more effort will be needed in getting all your data off Google - Your historic emails will need to be exported (and imported into iCloud if you want them) as well as your files in Google Drive. Other things like Youtube channels etc. may all well still be linked to those Google accounts even after the domains are switched over. They will exist (though no longer linked to the domain) until you delete / remove them, else Google will start charging you accordingly.
I moved my family (5 accounts, 3 domains) from Google to iCloud back in April - It took about two weeks of work to ensure all the historic data was transferred across before I delete the google accounts and avoid billing.
https://takeout.google.com is a valuable tool to help with the exporting.