which suspension seatpost for a granny who wants a comfortable ride while cruising the neighborhood
I'm afraid questions seeking product advice are off topic here:

https://bicycles.stackexchange.com/help/on-topic

Questions seeking product, service, or learning material recommendations are off-topic because they tend to become obsolete quickly. Instead, describe your situation and the specific problem you're trying to solve.

However, my advice would be to investigate the possibility of running larger, higher quality tyres at a lower pressure before experimenting with suspension seatposts.

Larger tyres, with more supple casings, run at lower pressure conform to the surface much better and greatly reduce the impact of road imperfections.