If you have the room I would suggest a ducted System for better air flow and temperature balance.

If the tenants are not blocking air flow, usually the problem, and the temp is below the thermostat setting by more than a couple of degrees call in a HVAC specialist to balance the system.Never allow tenant to have space heaters, they cause too many fires.Put all tenants on M2M and see how it goes for the rest of the winter.

No bugs or diseases inside, plenty of air flow.

Check the heat reflective colors because over an existing roof you get much better air flow and a much cooler attic, thus lower cooling bills.

I have seen too many bathrooms with poor ventilation and the resulting mold/mildew/paint flaking from sheetrock, etc.Again on the older homes, I typically wind up adding a turbine or two to the roof to improve the air flow and lower the temperature in the attic.

Restricted air flow across those surfaces increases energy consumption and puts unnecessary stress on the moving parts.However, A/C systems are sealed systems and should not have to topped up or recharged unless there is a leak.

I'm not sure how your units are setup in regard to air flow.

Very efficient.Chances are the older system has substantial duct loss and should be tested.Mini splits have their place but cold air is heavier and drops so the bottom floor doesn't make sense.Electric motors do not raise in load consumption as they age, it's the mechanical work done that raises load, typically deferred maintenance on belts, shives and pulleys.I'd disqualify HVAC companies by asking; Are you proficient in the new Manual J,S & D standards (that will be required in the industry moving forward)These companies will understand laminar flow, stratification and static airflow imbalances that have a great effect on efficiency, operation and longevity of a conditioning system.Get 3 estimates, compare scope & make sure its permitted.

Airflow in radiator heated homes is negligible.There was a lot of unseen damage.

In Florida, I recommend them as it is very hot here and it's best to have air flow in all rooms due to the climate being conducive to mold.