Exchange Online to retire Basic auth for Client Submission (SMTP AUTH)

I'm trying to prep for this and struggling a little bit.
1) ACS: I can't figure out what approximate costs to expect. Looks like I need to set up a subscription, and Azure Resource Groups, and the subscription will be free for 12 months, but after that some (but not all) features will be free, others will cost, and I don't find any named features that seem relevant to email. So, ???

2) HVE: As this is internal only and that won't work for my clients I haven't read up on this at all yet.

QUESTIONS:
- The well-known Microsoft page to set up multi-function devices - ( https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365#smtp-relay-configure-a-connector-to-relay-email-from-your-device-or-application-through-microsoft-365-or-office-365 ) describes the SMTP Relay option. Will this option work using a static IP address, after Basic Auth is disabled?

- Will an IIS SMTP relay from modern Windows Server servers be able to authenticate using OAuth, if we need to go that route?

Thanks,

Steve