I just sent an email from my MacBook using the new alias and checked my network, the request was sent to p60-smtp.mail.me.com. That is just a CNAME for smtp.mail.me.com so the standard SMTP server.

That is a good point. So far I've had issues with third part apps and logging in when using custom domains. They don't seem to like using the custom email address as the username and have no functionality to accept an custom alias. So far I've only used iOS Mail and the beta.icloud.com website to send email which works well.

Same issue here. on my iPhone 11 Pro the "Other Data" is around 90-130GB as it seems to vary. On my iPad Pro 2018 it's about 65GB currently, but that gets used a lot less. I've tried backing up through iTunes and syncing, but nether made much of a difference. I have noticed that iTunes says my iPhone has 169GB free, when the phone itself says there's about 20GB free with the "Other Data".

iTunes reports 169GB free for me right now, but on the phone it says 236GB used of 256GB.

Yes! It's been varying between 90-130GB. Tried connecting it to my PC and doing a sync but that hasn't helped.