I can’t speak to the overall safety one way or another but for many years that was the only way to have an Apple ID, I used an @mac.com email for iCloud until they permitted changing your email address few years ago, now it uses my gmail address as you are doing.

As for safety, no matter which you choose, make sure you have 2 factor of some kind enabled on both the email and your Apple ID, that will make a bigger difference in my opinion.

I switched mine from Gmail to my iCloud mail years ago. Absolutely no problems.

Yes, it works just fine, just make sure you heed the backup instructions (like with any online account like this) if you are worried.

I’m pretty sure I have read that after you transition your Apple ID to an iCloud login that you are stuck with that.

I have been considering moving as well because Gmail doesn’t work well with Apples mail app, but I’m not sure I want my Apple ID permanently stuck to iCloud.

Apple ain’t going nowhere

If you have iCloud+, I highly recommend buying your own domain (depends on the exact domain, but a lot of TLDs are around the price of iCloud 50GB) and set up iCloud Mail to work on that custom domain (this is a feature included with iCloud+). That way you're under control of your mail address should you ever want to switch away from iCloud, and don't have to update your address anywhere ever again.

thanks all for the answers, I switched to @icloud for my AppleID. It indeed said that I won't be able to switch the email back to other if I will do this, but if I will ever want to get rid of @icloud, I will probably as well will be getting rid of all the Apple services :D