Choosing keywords is going nowhere

Good place to start. The one thing I'd add is to group them before writing anything. You'll often find five queries that all want the same answer, and that's one page, not five. Splitting them up means your own pages end up competing with each other.

start from what your old agency already ranked you for. search console has every query you get impressions on, sort by impressions and look at the ones sitting in positions 8-20. that list tells you what google already associates with your site, which beats any brainstorm.