No you can totally modify mail headers anytime you want to, just be prepared to get mail rejection if you're not following current mail security best practices.
I'd recommend just renting a cheap vps from vultr or something, then you can setup your mailserver to send from anything you like. That's how my mailserver works. I pay like $3 a month, and its plenty of space for a single user mailserver (i have like 3 mailboxes)
I did go through the work to setup dkim/dmarc/spf. Took a weekend, but wasnt too bad. My mail is received by gmail yahoo and Microsoft. I imagine doing the same with onion addressing would be complicated.
Invincible. The comics are great, but I think the show dramatically improves a couple characters