Domain Transition and Email Setup Expert

Moving my main work email from one of my domains to another. I run two brands, each with its own domain, both on Google Workspace.

What I need:

1. Review the current email and domain setup across both, tell me what needs changing for a clean transition.
2. Set up the new address properly in Google Admin (domain and email records, routing, aliases if needed).
3. In ActiveCampaign, update the verified sending addresses so the new address on Brand A and the general address on Brand B are the ones used. My old address on Brand B should be removed as a sending address entirely.
4. Check authentication records are correct and verified (SPF, DKIM, DMARC) for both sending domains.
5. Flag anything else that needs updating as part of the move... other integrations, forms, automations, anywhere the old address is hardcoded.

Current stack: Google Workspace, ActiveCampaign, two domains.

Looking for someone who's done this before and can explain what they're doing in plain English. I'm not technical on the backend side.

Please tell me roughly how long you think it'll take and when you can start.