They get through a lot of wood already keeping the stove and a small open fire going. However they are getting on a bit so this might get harder in a few years. How thick were your walls before you insulated them? I’m thinking if they just got double glazing put in it would make a massive differenceInsulation, double glazing and draught proofing are the first major steps to take.
I just put 50mm insulation on the inside of all external walls in my ancient, cold and draughty farmhouse. I also fitted double glazing and insulated the loft with a generous amount.
House now runs entirely on a log fired range cooker in the kitchen/diner and a 5 kW log burner in the living room, temperature wise you can comfortably sit in your underpants should you wish to. We do have a new oil boiler too that goes on whenever the fire hasn't been lit but that isn't often. Been in almost a year now and haven't bought more oil yet. We do burn a lot of logs which could be a problem for your parents depending on how fit and well they are. The deal in this house is I will always make sure there are dry logs, missus can burn as many as she likes.
Insulation should be the priority.