As you already have Solar, a small Combined Heat & Power (CHP) system might be something to consider.
There are a good number of calculators online.
Basically they are as efficient as a furnace, but
they give you free electricity (to the solar batteries) during those dark cold times when the panels are NFG.
Here's a silent, fuel cell based
5KW Electric - 7KW Heat unit:
https://metaconsa.metacon.com/5-kw-c...er-system-chp/
Noise is likely an issue for piston engine based systems.
I don't know that the makers of these units are aware of just how effective Quarter Wave and Helmholtz Resonators are for constant rpm engines. (exhaust and induction)
Quarter Wave Resonators are simply a tuned length short piece of pipe, T'd into the exhaust and capped at the other end. = cheap.
There are calculators online for them too.
IIRC the engine based CHPs need a service every 14 months if they run 24/7.
That would be like every 10? years for your use case.
The combined heat and power plant in a single-family house — what you need to know
Micro combined heat and power