We have a 1300sf 1950s single story 3-bedroom house near Sacramento, zone 3, we want to improve our energy efficiency and electrify (tho we have too much tree shade for solar). The house originally had a nearly flat tar and gravel roof over an exposed beam tongue and groove celling. In the mid-90s we added central HVAC plus a new higher peaked shingle roof to accommodate the ducts and gas furnace underneath  – so there is now a shallow attic/crawl space between the old roof/ceiling and the underside of the higher peaked roof. It gets extremely hot in that attic space in the summer (140!) – and summers here are getting hotter and hotter. We are evaluating two proposals for which the total cost is about the same.
1. Seal/insulate the underside of the current roof with spray foam insulation and eliminate the attic vents making the attic conditioned space. Then use our existing duct system with a heat pump and an air handler. (Tho one room, formerly the garage, would need its own mini split).
2. Seal/insulate the top of the old lower roof/ceiling, keep attic vents to the outside, remove the ducts and the ceiling vents and then put ductless mini-splits in each room. (A significant part of the cost of this proposal is to remove the duct system and old insulation over the ceiling.)
We’re leaning towards the first option, to seal the roof/outer envelope and use the existing ducts with an air handler. Adding a mini split to each room seems like overkill in this little house. BUT we’ve been advised that the mini split system is more energy efficient since we wouldn’t be heating/cooling the conditioned space of the attic nor have heat/cooling loss through the ducts. Plus we could adjust the temp in different zones. It’s not clear to us how significant the energy loss with the duct system would be versus the mini-splits and it would be good to know what the trade-off are before making that decision.
Could you advise us what further questions, considerations or research we should do before making the decision of what type of system to install?