Solar radiation will always be the most difficult data to acquire. Firstly, the equipment to measure it correctly and accurately is expensive. You can pick up cheaper units that will swing wildly throughout the day, and may end up providing useless data. The other way to get the data is mathematical deviations from a normal, or mean/average. You can use a small solar panel with a good specification and decay rate for watts/meter over time. Then infer solar radiation using charge rate.
The solar panel, or photovoltaic cell method is what most home based weather stations use. So you will only receive data as good as their calculation. I say do that part yourself, then you can tailor the calculations and assumptions to your specific needs. As far as working out the solar radiation, most of the methods are available online, and I encourage you to research which would provide exactly what you are looking for. Once you’ve found what you want to use I will be more than happy to help you with the calculations.
The biggest piece of advice I have is really think about how accurate you need to be. Remember, keep it simple, you’ll have much less headache, and an easier to use product. Grass is quite forgiving, it didn’t always have people around to help it grow 😀 Mine probably hates me because I’m always testing on it.
I honestly think you have gone much more into detail than I ever worried about. A lot of people will never understand the magic part of the code in the little box, they just want it to water, and keep things alive. You have done quite a bit of work and it shows, keep it up, I look forward to seeing what you are able to achieve.