As an FYI you can have the "vanity domain" if you just use the webfinger protocol with your own custom domain. Don't necessarily need to self-host it.
> As with LOCAL_DOMAIN, WEB_DOMAIN cannot be safely changed once set, as this will confuse remote servers that knew of your previous settings and may break communication with them or make it unreliable.
https://docs.joinmastodon.org/admin/config/#local_domain
They also recommend a "301 Moved Permanently" implementation.
You can always host a lightweight ActivityPub server like honk (https://humungus.tedunangst.com/r/honk).