I think this is still in play though for a future time.
I was comparing the cost/benefit of outfitting the rest of my house with mini-splits vs doing some kind of central setup. I think the central setup will be more cost effective.
Mini-split setup;
I just added the 18k BTU single zone to the great room. Then I would add another 12-18k single zone unit for the living room/dining room (and kitchen kinda). Then for each bedroom, a 3 zone 27k unit with three 9k heads. Total cooling capacity: 4.75 to 5.25 tons. This is on the high side of what this house needs (probably closer to 4 tons). Cost of just the mini-split condensers, air handlers and line sets: $5,636 (before tax, shipping and the electrical work). That does give me a total of 5 zones which is cool but that seems like a lot plus a lot of additional installation work.
Central setup:
Probably a 3 ton condenser and a 3 ton coil. For a condenser there are many options, both A/C only and heat pump. In fact I can get a 20 SEER Mr. Cool condenser which appears to be the same type of unit as a mini-split. Then add a coil to my furnace and figure out the controls and ductwork. Plus lines. Electrical. Condenser options range from a Mr. Cool 20 SEER 3 ton unit for $1,671 (or $985.57 scratch and dent) to a Goodman 16 SEER for $2,190 and A/C only options are cheaper. A 3 ton Goodman in a large size is $438.00 and Mr. Cool options go down to $392 or so. Again this would need extra stuff. I don't know if I could use any TXV or if it is a good idea to mix a Goodman coil with a Mr. Cool condenser. Anyway, coil + condenser ranges from $1377.57 to $2109. Even with the extra stuff this is going to come nowhere near what the mini-split setup would. But it also only has 1 zone (plus the existing great room zone) and is probably less energy efficient. This wouldn't be a true DIY option but I could make it work if I could figure out ducting (and I would have the entire spring/summer/fall to have the furnace ducting apart). Realistically this is all I need. The rest of my house is pretty open. I would also tend to close down the great room ducts in the summer and open them somewhat in the winter. I say that because I expect to want some oil heat in the great room. This would be regardless of this option or the additional mini-split option.
Bonus:
https://iwae.com/shop/3-ton-goodman-eva ... 10471.html
This Goodman looks like it would fit perfectly on top of my furnace!
For rough numbers I think it would be safe to assume the central option should be a solid 50% of the price of doing additional mini-splits. That's because I'm capitalizing on my existing air handler and duct work. Plus only doing one more electrical run. One line set.