You can send email from your alias with the Mail app, I do it every day.
You need to set it up manually (not use the certificate from fastmail).
After you set up the email address, go to Settings -> Password and accounts -> Tap your account -> Tap you email -> tap the "Email" field and then you can add your aliases tapping the button "Add Another Email"
From Safari 12, only Safari App extensions will be allowed, installed from the Mac App Store.
No more self signed extension or Safari Extension Gallery.
Version 3 will add more feed types: Podcasts, Mastodon, Twitter/X, Calendar, Reminders, Weather, Finance tickers and more.
It will have a new UI, new features like notifications and local transcripts and summaries and many quality of life improvements.