I'm the wrong person to ask, as I'm fairly new to this. This is my first foray into home automation in general, but this platform has a Home Assistant integration that I'm playing around with. But the included base station is what is aggregating all of the data from the sensors and sending it out to the cloud when connected to wifi. HA is able to poll that base station for the data as well, and I think you can do that purely on the local network. You can keep all of the data local to only the base station and not use their cloud service.

I picked this platform because it has a lot of out-of-the-box functionality as I'm just getting started. It looked like I can still play around outside their walled garden as I do more home automation stuff. Beyond the outdoor weather data a site like WU wants, it has sensors for indoor temp and air quality so I can set up some HVAC routines or monitor if my chest freezer for getting too warm. It has a soil moisture sensor and one day I'd like to set up an automatic irrigation system for my garden. I can expand the connected sensors as I think of more things I want to try.

I think I could have accomplished a lot of the same with bare sensors like you're suggesting but I wasn't ready to take that on. That was too much learning curve for me right now.