Well first welcome to home automation, and second - as advanced and fun as this is, remember we’re still VERY early in the tech as a whole. Cross platform device support (while good in HA, better than most platforms) still is one of the industry’s sticking points. You have the right idea sticking to verified open platforms - but you have to ensure that the protocol is supported in HA and even then, you’ll STILL need to check the idiosyncrasies of specific devices and even integrations. For instance, you can use that Sonoff Zigbee coordinator with BOTH ZHA and Zigbee2MQTT and device support is slightly different based on your choice. (Read: you’ll have to research everything before purchasing)
Good first start on the device, but you also chose a very hard to support protocol:
That device is Zigbee- but it’s Zigbee Green Power - so it’s a special animal. Follow that link and maybe Hedda can help you understand the state of ZGP in Home Assistant. Without that you’re probably falling back to the instructions for Legrand+ Home control - but you’re right those instructions are a bit of a mess.
To summarize the link (and last post from Hedda was about a month ago - even if your device to pull away from the Legrand device - you’re going to have a tough time finding supportable ZGP devices at all (see previous link)
Since you don’t want WiFi devices (that’s fine, I’m not a wifi IoT person either) your choices are basically Zwave, Zigbee (not green power in most cases) and Matter over Thread. Matter will be the emerging standard you want - but it’s brand new and device support is still spotty. Personally, if I were starting NEW today, I’d go Zigbee / Matter - preferring Matter versions of devices as availability allows.
For suggestions on SPECIFIC devices, we need your geography as device availability and power requirements vary WILDLY by region.