Looking through the Kidde Product Portfolio I found some products which could create an interesting setup:
They have both AC and 10 yr Battery Combo Devices that inter-connect wirelessly on their own network
So my thought is, connect the Master (Co-ordinator) as an AC wired Unit*, with the SM120X and CO120X (& EcoLink Switches) directly connected to that; then the Satellite devices will be Battery Devices located in the Bedrooms.
My understanding is that the Satellite Units should still wirelessly trigger the Relay Modules via the Hard-wired Co-ordinator Pigtail. (just waiting for confirmation from Kidde Support to validate that, but intuitively appears to sound like the case)
Interconnect Capability
This model has AC hardwire AND wireless alarm interconnect capability. When one hardwire or wireless interconnect unit sounds an alarm, all other compatible hardwire or wireless interconnected units will also alarm.
*When the House was built, they included 3 hard-wired Smoke Detectors - however NONE of which uses an interconnect wire and NONE of them are in any Bedrooms!
(currently the bedroom units, to meet code are just independent Battery Modules plus a couple of independent 10 yr CO Modules)
Indeed all three AC Smokes are all within the same ‘open’ space (two in downstairs Hallway and one at the top of the stairs) So I’ll just use the most centrally located of those for the Co-ordinator, then Battery Units for each of the 4 bedrooms.
I can leave those other two hard-wired units as just ‘dumb’ modules although could replace those too - right now these devices are quite reasonable - on Amazon, $55 for the AC Module and $79 for 2 x smoke only; the combo smoke/CO battery are $61