Yes, indoor triangulation and tracking is not quite what beacons were intended for, they are optimized for running on battery, and as you mentioned correctly, you need to maintain them (exchange batteries etc.). It’s likely hard to build a good navigation system based on this, at least I have not heard of any such in existence, let alone open source ones.
As you say you’ve got WiFi coverage, I suggest you check out FIND.
There’s a link to client implementations here, including the Github hosted source of the client app (note it’s Android only per default due to some restrictions Apple has builtin into for iOS apps to access WiFi data). The default app can be used to ‘learn’ the buildings, and you can include a client code part into your own app to make it query the FIND server that provides access to that previously-learnt building data (it’s essentially simply returning the closest Point of Interest as defined by you when learning).
You may or may not use the openHAB integration documented in my link, it would work without, too.