FWIW, a $4/m Exchange Online Plan 1 is probably enough. Or even the $1/m Exchange Online Protection if you don't have any need for a hosted mailbox. Anything that'll get you ongoing access to the Exchange Admin Center.
I use a single M365 Business Basic account, as a conventional mailbox, for one of my domains. From within the Exchange Admin Center there's extensive control over mail flow -- domains to accept mail for, inbound and outbound connectors for routing mail between on-prem mail servers. Best as I can tell, literally ongoing any subscription that gets you an account with access to EAC ought be enough to route any or all of your email through EO in either direction.