How to control output resistance with voltage
There are digital potentiometer ICs available that you could use in place of R1. You have to communicate with them using a micro-controller or other device however. Here is one example:
https://www.mouser.com/ProductDetail/Microchip-Technology/MCP4132-103E-P?qs=hH%252BOa0VZEiBueXVavvhYaw%3D%3D&amp;gclid=EAIaIQobChMInJrjj7jh6gIVjJ-zCh1IrQoVEAQYASABEgLkdfD_BwE