I’ve done a ton of research trying to find electrically flow controlled valves, and it took a long time for me to find an option that wasn’t in the $250 and up range, with almost all being in the $500 and up and “industrial” grade.
BUT, my work seems to have paid off. The best option I have found so far are valves like these: https://www.amazon.com/dp/B0119ZI38U?th=1
I haven’t purchased one yet, but I think these should be able to be controlled using PWM to slow down the actuation speed to get more control of the position, and coupled with a flow sensor gives the feedback necessary to adjust it well, and even makes it possible to adjust for changes in pressure. Unfortunately I can’t find ones that are reasonably priced that work with 24VAC like regular sprinkler valves, so they will require a separate power supply or a voltage converter.