there anything I can do to guard against that issue in the future? Is there a way to mirror an account between two instances or something?

Open an alt, export/import your settings

It's possible to export your Subscriptions, Community/instance blocks, and saved posts/comments to an account on a different instance by exporting your account's JSON file, which you can do in your Account Settings page.

You can then import the JSON file in that same area on your alternative backup account, so that your experience is uninterrupted. Personally I would recommend making a backup account on Sopuli.xyz or Lemmy.cafe, they'll give you the most similar vibe due to how their instances are set up.

I have alts. No automatic sync, but you can list your alts in your profile like I do.

On mobile make sure to use a client which makes it easy to switch between accounts (I use Thunder, which additionally shows server status in the account switcher!)