I’ve been hesitant on adding support for soil moisture sensor. To be fair, I did some research on existing soil moisture sensors and even reverse engineered a few (https://rayshobby.net/wordpress/reverse-engineer-a-cheap-wireless-soil-moisture-sensor/). Your point 1 is definitely valid: there are many soil sensors on the market and many are quite rubbish. Those that are good also don’t use any common standard, some are analog, some are digital, so it’s quite difficult to pick one and say that’s the one we are going to support. Moreover, ideally the soil sensor should be wireless (dragging a long wire from the lawn to the controller is very inconvenient), and for robust sensing it should probably be sampled at multiple locations instead of just one location (e.g. what if the spot the sensor is plugged into is particularly wet or dry compared to the average?). Introducing multiple, wireless sensors increases the complexity of the system and decreases its reliability.
Your point 2 is also valid but not a show stopper — we could implement some sort of scaling so that the watering percentage gradually reduces when the soil moisture increases. Also, there are threshold switches which basically convert analog signals to digital based on a preset threshold.
Your point 3 is basically what we ended up with.
So I am still unsure what to do about soil sensor. We would like to add some initial support, but I also don’t want to introduce a solution that ends up wasting people’s times and turns out garbage. Probably the first thing to decide on is a small set of soil sensors that are of good quality and commonly used, so that we can start taking a look and see what needs to be done to get them supported.