“There is nothing magical
“There is nothing magical about minisplit heat pump systems having more heating capacity. They do it by having a variable-speed compressor that you can “over-speed” at low ambient heating conditions without overloading the motor. Most U.S. manufacturers of variable-speed compressor heat pumps for ducted split systems have the same feature with the same benefit. It just costs more than the lower efficiency equipment.”

A really MAJOR difference with cold climate mini-split capacity and efficiency is the use of vapor injection scroll compressors where the vapor injection (not just the compressor speed) is optimized for greater capacity & efficiency within the targeted temperature bands. This is WAY more than mere modulation of compressor speed.

SFAIK there are still no US vendor of ducted split heat pump products using that type of compressor technology (though Carrier has at least experimented with it using prototype Copeland vapor injection compressors.) It’s almost as if the US manufacturers are ceding the cold climate heat pump market to the Asian vendors, who are already on 3rd and 4th generation products using that technology, and not just in ductless mini-splits. There are several VRF full sized air handler options that can be paired with cold climate outdoor units from several vendors, just not any from US vendors.

“how *do* mini-splits handle
“how *do* mini-splits handle reheat during defrost, anyway?”

Mini-split defrost cycles are normally done by reversing the heat flow, pulling heat out of the house and into the outdoor coil while defrosting. That is normally done with the indoor unit’s blower at it’s lowest speed, with the vanes directed downward (for wall-coil types) to minimize the potential comfort issues of blowing cold air during the heating season.

Yeah, existing hardware can
Yeah, existing hardware can skew things one way or another when you’re not starting from a clean sheet, of course 🙂

Apologies for my own
Apologies for my own overharried math! My prior post should state that the heat pump is apparently needing an extra 12kBTU atop its 18kBTU capacity, not 22kBTU!

The main reason for going
The main reason for going with either hydronic or forced air furnace is dependent on what is already in the house. If there is an existing hydronic system with a separate ducted cooling system, it is best to utilize a heat pump through the ducting system as the primary source of heat while using the existing hydronic system for auxiliary. If there is no existing ductwork, it is practical to consider Mini-Split heat pumps for cooling and primary heat source. We need to take every situation on its own merits. There is no one better way.

Yet another reason to go with
Yet another reason to go with hydronics instead of a forced-air furnace for backing up a heat pump with gas, I suppose? 🙂

For residential purposes, a
For residential purposes, a heat pump system with the largest indoor matching coil that is properly sized for cooling, utilizing a variable speed blower set to below 300 cfm per ton, is going to handle most humidity issues. For ROI purposes, it is beneficial to just use a 5kw bank of electric heat that is already in the system to cause the cooling to system to operate longer cycles for extreme dehumidification times. With a standard cooling system using a psc blower motor, I’ve even built a fire in our wood stove on low heat, high humidity days to dry the house out.

While you’re correct that
While you’re correct that balance point isn’t going to matter much in Florida as that’s a heavily cooling-dominated climate, I seriously question your sizing advice regarding strip heat. Electric duct (strip) heaters put out 3413 BTU/h/kW, so you’re basically saying that for the OP’s house, which can be heated by a 5kW strip heater (slightly over 15kBTU) by itself, that the heat pump (a nominal 18kBTU unit) is going to require another 22kBTU (at a minimum) to be fed to it to defrost the outdoor coil? That sounds like a fair bit of an oversize…how is that heat pump running over nominal cooling capacity during defrost operation?

Sidenote: how *do* mini-splits handle reheat during defrost, anyway?

Would a heat pump or air
Would a heat pump or air conditioner with a hot-gas reheat function (AFAICT from manufacturer docs, AAON offers that feature on their CB/F1 HP/AHU pairing) be a sensible alternative to a separate dehumidifer in this circumstance, or is the extra refrigerant piping and valves for HGRH considered a prohibitive level of complexity for a residential application compared to having a standalone unit?

There is nothing “magical”
There is nothing “magical” about anything in the HVAC industry; it’s all a matter of physics. Physics dictates that heat energy utilized for condensing water vapor into a liquid cannot also be used to lower the temperature of the air at the same time. When energy is used to remove heat from the air, that same energy cannot also be condensing water vapor in the air at the same time.

Other factors that allow Mini-Split systems to heat (and cool) more effectively is a lack of loss/gain of heat through the ducting system.