If you want more points downrange, you design your scanner to work on a smaller FOV. Some things change - you need an optical design that scans a small area in front of it somehow (faceted mirrors are popular), and at some point you need a laser source of higher quality, possibly moving away from diode lasers and towards things like fiber laser sources. This isn't conjecture BTW, this is what players like Waymo do and is evident from studying the sensor package if you're curious.

Roughly like in this paper https://www.semanticscholar.org/paper/RadarScenes:-A-Real-Wo...

I reckon it's probably not that bad, there are big surfaces that are almost normal to what would be incoming radio energy. Stealth shapes tend to reflect energy in a completely different direction from the source.

I think "stealth" planes assumes the radar is under the plane on the ground? For the geometry. And they have some color or alloy that reflect less.