Do you want free email for multiple addresses on multiple domains with all the features of a Gmail? Just setup Cloudflare to manage your email, create rules to forward to regular Gmail accounts, which you can configure to send email from your domains. 100% free.
just forwarding is great if you can afford losing mail. Gmail will drop your messages occasionally.
You can use gmail's own relay for sending but it won't DKIM sign which is a must even for Gmail itself.
for a more reliable solution use forwarding AND POP3 fetching with some provoder OR use https://gmailify.com which offers own relays too for ~$7 a year.
The usual ones like SendGrid or Mailgun are probably not great for personal email, but it could probably work. Personally, I’ve used Duocircle for a similar setup and it worked pretty well: https://www.duocircle.com/email/outbound-smtp