I certainly dont have any foam insert experience, but I do have a lot of road and a reasonable amount of offroad experience carrying stuff, but 20lbs of stuff isnt that much, and really, good tires with reasonable pressures and being careful riding, things will be fine......but----some people cut sidewalls a lot, and some people ride over all kinds of crap without any awareness of what they are doing--so really, so much of it depends on how you ride.

I am hoping to do a bikepacking trip this year in an area where it will be somewhat remote, and all the stuff I've written down is pretty much what I'll be keeping in mind--specifically, being careful, watching and feeling what tire pressures are good, and being conservative when there is chunky, sharp stuff--ie, getting off and walking for a bit rather than risking cutting a tire (touch wood, touch wood)