Thanks for posting this, that’s a clever workaround! If you’re up for giving Anki 2.1.50 a try (not all add-ons are compatible yet, so beware), I just released v1.4.0 of IOE which should fix the auto-canvas-fitting: Image Occlusion Enhanced [Official Support Thread] - #53 by glutanimate
The fix is a bit hacky by necessity and I was only able to test it on a limited number of set-ups so far, so I would be interested to hear if it works for you.