Someone mentioned getting around mesh and rebar. Only outside forms go first. Then plumbing goes In as plumber puts in the pipes before rebar and mesh. Sealing ends of pipes. Plumbers cap it with PVC caps then cut them off. You are not doing anything different other than moving dust and hair instead of water and poop. Poured walls? I assume you need perfect placement. Perhaps you can build a small form around where your intake vents go to to keep cement out and give you some room to trim pipe. I’m not a vacuum guy, just going off what I see walking through houses. Crush won’t happen. The soil should be compacted. The cement does not compact the soil further, at least noticeable. Little bit down you dig will not pose an issue. Plumber does it everyday. Your vertical runs will not have weight on them anyways. If worried about under slab runs, put it in schedule 40 or use schedule 40.