Home assistant is great for home automation stuff like lights and cameras and AC and such but for controlling your water system you might want something less complex / more purpose-designed. There are many arduino-based projects for doing exactly what you’re looking for - https://arduinogetstarted.com/tutorials/arduino-automatic-irrigation-system or https://www.instructables.com/Automatic-Plant-Watering-System-Using-Soil-Moistur/ are two i just looked up real quick. I’m sure it would be possible with assistant as well though if you just wanted to centralize.
doesn’t seem like it needs to increase the electric usage by that much, you only need a RasberryPi or equivalent to run the server, and you control everything with your phone… the outlets and solenoids shouldn’t need much power to do their thing. I saw earlier today how many options for EVs are available in South America (anywhere outside the US imperial core, actually) and it was a bit shocking. The conversion to EV business seems like a great idea - especially if they can standardize replacement parts that would be amazing.