So Affinity Designer, as it turns out, has a lot of potential for cel animation work. You can create a decent enough animatic in the bitmap persona using layers for onion skin, then just stick it all together in your editor of choice.
My only request - could we have an "export selected layers as PNGs" feature please? That would save me having to go through toggling transparency and exporting one-at-a-time.
animatic_scene2.mp4