This tool is great, I agree.

Having lived in NYC for years, I can relate to the shift. You'll get used to the lower density though.

I don't know if 15k is a good threshold personally (too low) and maybe you'll discover that by looking at census tracts and Google street view that you could settle on a neighborhood with 30k for example.

Also, I wouldn't focus on single family unattached. There was a Washington Post article from a few years ago that showed housing typologies per city and Philly I believe has both the lowest proportion of single family unattached (even lower than NYC) and the highest of single family attached (either attached on both sides or just one). We're a row home city, there really aren't that many apartment buildings comparatively and a lot of the existing apartments are in carved up row homes. Even if you don't like townhomes (attached on both sides) maybe consider twins, which are homes that are attached on one side only.