[jamesbr]

I'm in a similar situation re: moving off Google services.

I use Mylio (https://mylio.com/), I think it hits most of your points.

Works great for managing my ~200k images. Syncs between my laptop/desktop/phones and also to Google/Amazon drive with optional encryption.

You can define which size of photos are synced to various devices, so my phones have thumbnails, my laptop has preview size photos, and my server/workstations have full copies.