Hello Ldameron. I saw this question asked somewhere else, and am glad you came here so we could help you.
The lower your well pump operates at, the more GPM it will produce. When they say 21 GPM pump, it means 21 GPM average. That pump can probably produce as much as 30 GPM at lower pressure. It also depends on the depth to water in the well. At the depth of your well and using a 40/60 pressure switch with a 50 PSI Cycle Stop Valve, that pump maybe only able to produce 18 GPM.
So with 16 GPM going to the heat pump and another 3-5 GPM going to a shower, you maybe getting 21 GPM, but only at 20 PSI or so. The heat pump doesn’t care that the pressure is low, all it needs is the 16 GPM flow. It is only in the house where the pressure needs to stay up.
All you need to do is add a jet type booster pump to feed the house. It won’t even come on if the heat pump is not running. Water will just get pushed through the booster pump by the well pump, and you will have 50 PSI in the house. But if the heat pump comes on and the shower pressure drops to 35 PSI, the booster pump will come on and the shower pressure will quickly rise to a strong 50 PSI. With another CSV on the jet pump, the shower pressure will stay constant at 50 PSI, even if you are in the shower for a month. And with this two-pump set up, the shower pressure will always be 50 PSI, no matter if the heat pump is on or not.
See this link for a picture and better description. Let me know if you have questions.