Would an internal relay domain work without Active Directory trust

You can achieve this by setting up contacts in the source domain with a targetaddress attribute of the target primary SMTP address, or you can do it by configuring the domains as internal relay. I'd recommend dedicated send/receive connectors as you can customise the delivery and look of the messages a bit more, and you will have to have one if you go down the internal relay domain path otherwise you'll get stuck in a mail loop as the MX record will point to the target Exchange organisation. Generally when we see organisations do this they are pretty quick to get the networks linked, so once that's done you can configure the connectors to route traffic that way. As you say SPF/DMARC have to be correct, be careful if you are using DKIM, and make sure whatever message hygiene service you are using is configured appropriately as well.