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... (Budget: €50 - €80 EUR, Jobs: ActiveCampaign, DNS, Email Developer, Email Handling, Email Marketing, PHP, System Admin, Technical Support)