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But dropping from 2.4Ghz and 5Ghz down to 900Mhz does significantly improve the penetration from varying forms of obstructions and makes tropospheric ducting more likely to occur.

In my experience it's not the houses or trees that block you. It's always changes in ground elevation, a small rise here or there. Using 902-928 MHz (actually a 5 Mhz channel centered around 906) did not increase my range versus 2.4 or 5 GHz options at the same height above terrain.

And the fresnel zone of 900 MHz is much larger than 5 GHz so in it requires more height above terrain than 5 GHz. Not great. Tropospheric ducting is so transient and rare it does not matter for this application. And it doesn't really help getting your signal across town.

I regret investing in a bunch of 900 MHz transceivers, bidirectional amps, filters, etc.

Getting above the fresnel zone is another important hurdle to surmount. That’s at least 15 feet for a lot of 5GHz transceivers that are a mile apart.

This. We had good results bouncing LoRaWAN packets around 14km using directional antennas and decent elevation.

Watch your Fresnel Zone!