I Think you are going to need to manage the taco pump separate from the sump circulation pump. If you do things the way you have it pictured the water coming out of the exchanger is going to get way too hot, and likely to cause some serious temp fluctuations throughout your tank volume.
You want to be able to first engage the sump circulation pump, and if the temp does not climb or continue to climb to the desired temp, then cycle the taco pump for a short amount of time. Alternately you could have a 2nd temp probe monitoring the temp of the heat exchanger and if there was a call for heat and the temp was below a certain set temp cycle the taco pump.
The goal when heating the tank is a tiny, slow, even heat increases over a large volume of water.
I think you would be better off getting a small titanium heat exchange coil, that can go in to the sump and just have the taco pump to manage, you would want it to be super low flow, and even then I would put something on the line going to the coil to restrict the flow even more if things get too hot too fast. Even then you are going to have to carefully program the control of the taco pump and cycle it on and off for a short amount of time and then allow the water flowing through the coil in the sump to absorb the heat before cycling it again (if needed).
