To your multiple IMAP concept, I have been using isync / mbsync (name change, package isync in Debian) for years running via cron script to pull email from one domain at one provider and push it to a subfolder of another domain at another provider. You have to be aware of one specific gotcha but it’s otherwise been working all by itself forever without issues. Take note of the PipeLineDepth 1 for IMAP service providers which throttle your speed, I have to use it on the destination side provider config.