We use 26x2 slicks (Schwalbe Kojak) on our tandem which is good for hardpack but not loose stuff. We mostly do credit card touring and general riding on roads and, sometimes dirt roads.
Only time I would consider real aggresive treads is for riding in loose gravel or sand/mud. Too much buzz for my taste on hard surfaces.
If Panaracer Gravel King is available in 26", that might be a reasonable compromise.