Yes, land would be idea.
It would also be fantastic if these things had a Laser Pointer that measured distance directly in front or to the side of them and transferred that to the telemetry. You would have live reading of how far you are from walls, etc. It wouldn't seem like alot of expense or weight if you DJI done it.
Yes you can set RTH to land only, BUT i don't know if it would try to reach 20m before it (Land Only), we probably need to summon Blade Strike to awnser this question, because i won't be the one testing this. But i believe you would be safe doing this, when i think of it DJI as a promotional video of a P3 entering a old factory without issue.
Also give it a couple years, maybe even less and we will see collision prevention automatic onboard systems, probably 3d camera automatic mapping systems(if DJI actually use 3d camera mapping systems fr anti collision because of this post, please consider giving me a share of profit LOL)....