https://www.downlitebedding.com/comforters/winter-usa-origin-duck-down-comforter-oversized-dream-naturally/

Have...very warm and pretty inexpensive.

I've never seen a comforter with higher fill power than 650, and I don't think it's particularly desirable. The amount of down matters a lot more for warmth.

The thread count also is not super important since you normally put comforters inside covers.

You just want it to be warm, robust, and not too heavy(?). I think this one satisfies those criteria.