Update: manually setting the 0x9 RegPaConfig register to values stolen from the Arduino sample yields similar RSSI values.
Update 2: Most likely the RFO output pin is just not connected on the TTGO module, so you have to use the PA_BOOST output. At least the RadioHead library and a blogpost mentions that Hope RF's RFM95 doesn't have RFO pin connected. The TTGO branded LoRa module has a metal shield on it, but has the same pinout and physical size so I assume it's the same design as well. Of course this isn't documented in the datasheets...
Looks like libraries I have checked so far use the RFO pin only for the lowest power settings, while that could be used up to +14dBm output power. But instead use PA_BOOST for everything other than <2dBm. Didn't make any sense first, but looks like it's the workaround for the not connected pin...
Update 2.5: tested with an RFM95W: same behaviour - as expected. RX works, TX with an RSSI in -100db at a short distance, but works better after changing the Pa Config register and switching to PA_BOOST pin.
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