I am curious: What made you switch from Protonmail to Fastmail?
1. Is it easy to set up the redirect from Gmail to the new account?
2. When I send email from new account (custom domain), how are you completely sure it is not going to be on spam? I send unsolicited emails to people in big corps (journalist asking for interviews/comments, not spammy :-)
Yes super easy. You can do this using Gmail's filter service. However, many services will also let you log in to gmail via POP/IMAP (OAuth for auth) and fetch the email from your gmail for you that way.
> 2. When I send email from new account (custom domain), how are you completely sure it is not going to be on spam? I send unsolicited emails to people in big corps (journalist asking for interviews/comments, not spammy :-)
I guess it's hard to know for sure, but I've not noticed any problems with this when the underlying provider is a well-known sender such as Gmail or Fastmail.
On sending from a custom domain, your mail provider walks you through setting up a few additional DNS records (for DKIM, SPF) that allow you to send from your domain through their servers. They maintain the reputation of their IPs and so you get their good deliverability.
For the .contact gTLD you will find them at https://donuts.domains/about/policies/ or over at ICANN.