Electric panel heaters will suit. They automatically come on if temperature drops to what ever settings you require. Why larch cladding too? Cost would you of preferred cedar?

Looks like an exciting project!

We have similar in our garden and opted for a small dyson which is fine for the space.

I would look at underfloor - make sure it is insulated underneath and with a thermostat.

I’m also in the middle of a garden office build and went for this brand:

https://www.electricradiatorsdirect.co.uk/ecostrad-allora-designer-electric-radiators/?gclid=EAIaIQobChMIyYHsjYKO9wIVR-3tCh2cCgBKEAAYASAAEgJI-vD_BwE

Electric radiators filled with thermoil, which retains heat even after it’s switched off. Haven’t installed yet, so can’t tell you how effective it is!

I’m also installing a single room heat recovery ventilation unit, as the room will be reasonably air tight and well insulated.