Heat is struggling to keep up

I have a geothermal system (NY) with a supplemental heating unit in the attic. A/C performance is great. But in the winter, if outside temp is below freezing, the air handler is running 24/7 and struggling to get above 65 indoors. House is 25 yo with good insulation (at least where I can see in the attic).

I'd like to be able to hit 68F indoors. And it seems inefficient for it to run constantly. Would it be more efficient to increase the heat capacity in the auxilary unit, so the air handler can cycle off for some period? Other suggestions?