Cool story! Was there a reason using existing LoRa sensors was off the table? YoLink and maybe others are cheap.
I haven't needed to change the batteries on a single sensor yet for multiple years. I did get 2 of their SpeakerHubs which also act range extenders, but it's completely unnecessary since the single hub is perfectly fine wherever you put it. I mainly use the SpeakerHubs as sirens, but they can also do text to speech, e.g. "(Loud Siren) Water Leak detected in master bath".
They've already saved me a couple of times.
I keep reading about all these standards for connected devices e.g. Matter, but in the end these just work perfectly for my use case.