Haven’t done it myself, but his might help you for further research
iptables
https://unix.stackexchange.com/questions/264589/traffic-of-specific-program-through-vpn
network interfaces
https://unix.stackexchange.com/questions/234583/routing-on-per-application-basis
Which is actually a good thing. Might sound scarry and counterintuitive at first, but is the right way to go.
Especially with a custom domain, (can be smth cheap for 2 bucks a year) and using aliases for all the different places… like onlyfans@mydomain com… this prevents other malicious services from sending spam to your main email adress, since you can just delete the alias.