Yeah not having a backup seemed like one of the sticking points for going all heat pump to get those rebates. My parents went with two condensers, so if one goes you still have the other. Something like that + cold weather heat pumps + a generator would keep you going through most any situation. Still have rising electric costs to deal with though.

If you have to keep propane in the mix, I'd go for the combo on-demand boiler, just based on my experience having a natural gas version. It cut our therms in half vs a standard older gas furnace, hopefully you'd see similar efficiency with propane. And if the minisplits are doing the heavy lifting on heat, your gas needs could be pretty minimal. There are still some valuable rebates available for a setup like that.