'Light' is easy. Walking men with everything they might possibly need in their rucksacks, in units without organic technical mobility assets. Focus tactical mobility and low signature, but with man-portable missiles and other systems very dangerous in the infight. And I think to maximize effectiveness such units have to be organized almost irregularily and very flat. Mobility and fire support assets as needed.

But then it becomes complicated, so lets start with the upper end. I think the real place these days for heavy mech inf is urban combat. Vehicles are rolling fortresses with protection far outweighing mobility. Bringing infantry directly and under fire to the close-in combat. Here also the assault gun would come into play, as part of the direct-indirect fires mix. Such units are way too unwieldy for fast open terrain movements and their standard effector portfolio not suited for long range combat. But equipped with guided missiles also very good for defensive operations over longer ranges.

And for 'in between' I'm not sure infantry is the right tool. Rather I'd think of cavalry. Focus on mobility (also air portable and amphibious - so let's keep under 15/18 tons) and (missile) firepower. Very important aerial ISR, primary fire support via air power, also logistics support primarily via air. Think VDV and think BMD-4. A reduced infantry contingent can be valuable for some tasks, but dismounted combat should not even be tried as it robs the units of its greatest assets speed and mobility. Designed for rapid maneuvers in rural and vehicle-compatible environment. These are the units for 'recklessness'.