Combining Per-Impression and backfilling?

Yes, but only in the Pro version (no alternative ad supported):
https://adinserter.pro/documentation/ad-impression-and-click-limiting

I see that I can limit by number of impressions, but how do I backfill that same ad spot?

For example, say I have an ad running after Paragraph 3 on my posts. I want to show an ad for 1000 impressions, then after that runs out, show a different ad, instead.

Can this be done?

Insert another block at the same position and delay showing for 1000 impressions.