Ditto the insulation and heat pump comments, plus I’ll add double-paned (or at least updated) windows to the pile. Re-doing our window from the 1952 originals to double-paned was a significant difference in how much we were spending and it made the house more comfortable at lower thermostat settings as well. Insulation came a few years later; heat pump was added this summer.

There are those dish heater things that Costco sells, too, and they are incredible for heating up a room and they didn’t spike the electric bill much, IIRC. Economically considerate space heaters might be your best bet until you’re able to get to the upgrades suggested here.