Zephyr provides an open source implementation of all layers of the BLE stack. The radios of some devices are documented extensively, Nordic nRF5 devices are probably the best example.
The Bluetooth SIG requires that you qualify your device if you advertise that you use Bluetooth IP, similar to what is required for the cellular space. Do you have to do this if you’re just “Bluetooth compatible?” Maybe not. Whatever the case you have to conduct FCC part 15C testing (intentional radiator).
anybody doing new BLE products has a 50/50 shot of using Zephyr in current year. I think the real benefit of Zephyr is that the wheel has already been invented, no need to do it yourself
Additionally, you aren't intended (for many situations) to use a single "main" Zephyr install, but to include what external modules you need in your project's west.yml. If you have a number of projects sharing the same Zephyr install that's a separate discussion but installing every possible toolchain/HAL is not the only way to do things.
RF dongles almost always are Nordic Semiconductor parts and they use their proprietary (but open) protocol called Enhanced Shockburst. The minimum latency of BLE is 7.5ms (fastest connection interval in the spec) while ESB can be in the 100s of microseconds. Not sure about power savings but latency is better.
Best option is PoE and battery back up (if they elect to pull the meter)? I guess they could also cut your cable. Or forget all of that and get a nice pup
the crustier EEs among us have been burned by downloaded symbols. It’s often better to make your own symbols and footprints unless you have a high quality source from the manufacturer. Even then, some symbols are not organized in a logical way. I personally make all symbols/footprints not in the kicad standard library.