Before you use Takeout, make sure you're not relying on mbox format being readable everywhere - lots of newer email clients want individual message files instead. Takeout works fine and it's free, but consider exporting to a format like PST (if you're going Microsoft) or at least keeping the mbox files in a tool that can convert them later. If you want something less technical, some email providers like ProtonMail or Fastmail have import tools that can pull your Gmail data directly without you needing to download a massive archive first, then you just migrate once you've tested the new setup. The nuclear option is just leaving your Gmail sitting there as a backup while you use the new provider - Google's not going to delete it unless you explicitly close the account, so there's no huge rush to get everything out in one perfect format.