Anyone added a Mr Cool to supplement their central AC setup?

Apparently our normal unit is a bit to small. Could add a Mr cool split unit to a couple of rooms, disable the normal vents to allow for more air in other areas from central unit.

Cheaper than a new full sized unit.

A mini split will work, especially in an area you can close off like a bedroom - - I would first see if you could use more insulation and sealing.

But you say your system is "a bit too small". If you just need a little boost - I would first check for areas you can insulate better or seal up.

Your attic may need more insulation. Some houses have no insulation over the garage. Adding insulation if it's needed is the cheapest way to reduce AC and heater usage. also check door seals and the caulking around windows and any exterior penetrations.

If your garage door is not insulated, even the Owens Corning door kits will help some. Maybe even add some of the window screens that help save energy.

The energy savings through better insulation are cumulative. I hope you can fix your issue by simply blowing in more insulation.

I would get a couple of opinions and also see if you need to balance out the system a little better with things like an additional return air. A mini split will cost a little extra each month and insulation will save you a few bucks each month.

There are many solutions prior to the next complete system design & replacement ... better tailored to your spaces.

I agree with your solution principles and have employed the simplest methods to achieve all that in the past with poorly designed & older central systems.

Even with a heat pump setup here - which is very sensitive to supply registers "vents" being closed off - it worked well to buy a simple window AC, a bit oversized for the hotter or more windowed rooms. This was the original "mini-split" and still today similar in many ways ... some come as heat pumps, many are seen under the window in motels.

Mine came from Amazon, LG and offers a DRY mode which takes care of humidity VERY well.
https://www.amazon.com/gp/product/B01D3FOMHK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Now sizing is critical and formulas can be daunting ... opinions crazy.
The unit above is 10,000BTU or 5/6 Ton
a) Nothing less will take care of space #1 = 150sf but 105sf of dbl.pane glass facing East
b) Same unit easily takes care of Master Suite, 393sf but only glazed with 15sf dbl.pane West + 12sf sing.pane South; note high humidity also easily quelled from long hot showers or whirlpooling.
$340 was a bargain and it was one-man installation in short time.

a) is not used much since the central system was properly designed/installed last summer
b) still employed to dehumidify in minutes or for very cool sleeping hours

If you are a renter, then take 'em with you
When the new system comes along, send one to relatives
In the rare case it fails, get your money back + replacement, no need to get a technician

OK, call it the redneck way ..... haha