I use it in tandem with proton, so the quantity of emails going to iCloud are less, however I’ve not experienced this kind off issue with either.

Generally if a service is going to have issues with SL, it’s at the point when you provide the email that it says it’s invalid (likely due to the domain being on a black list to prevent use of the service)

Leaving this here because it’s free and potentially solves your issue.

Snapdrop.net

It works similarly to airdrop and the only requirement is that the site is open on both devices on the same network.

I’m a fan of OTP Auth.

It’s been reliable, supports local and cloud backup / exporting, is simplistic in use and has a strong privacy policy.