There's also Modular Rocket Systems with a 2.5m quad NERV cluster, and Penguin high-ISP motor (with single, double and quad clusters): https://mods.curse.com/ksp-mods/kerbal/224105-modular-rocket-systems-stock-alike-parts-mod MRS is my favourite addon outside MechJeb or kOS
I am using MRS in a sandbox save, and can't find the cargo bays or MRS RCS components. Are there obvious gotchas I may have missed, or is this going to be a case of stripping out AddOns until something changes? I've searched through all the function areas, then searched through the size and feature lists. I have Behemoth, MechJeb, kOS, Reusable, Fuel Tanks Plus, MRS, Docking Alignment Indicator, Kerbal Alarm Clock and Community Tech Tree installed. Maybe there are known conflicts?