Seems a little low, especially when it gets pretty cold. Its something like 3.4 BTUs/watt, so you would be around 17,000 BTUs
I would do the same . If I was doing 100% electric resistance heat .Put another 5kw in it add a 2 stage heat tstat, control wire one 5kw for stage two, set tstat for 68 if the 1st stage cant maintain 68 it will pull in and cycle the 2nd stage to maintain 68.
Now you have the best of both worlds.