After some tweaking, I now drastically reduced the  offset (aka frequency mismatch) between my modules.

Without any matching, I had a deviation of approx. 1.500Hz. Now, with dynamically adjusting the frequency based on the reported difference, I am able to keep the offset around +/- 10Hz. That's an improvement by a factor of  more than 100!

Due to this improvement, I hope to make use of even lower bandwidths in the future.  Instead of the predefined ModemConfigs available in the RF95 driver,  Using setModemRegisters(), I've set my modules to a bandwidth of 15.6kHz, Coding Rate of 4/8 and a Spreading Factor of 512.  A quick look at gqrx confirmed the desired bandwidth and exact identical/matching frequency.

I am pretty curious how much this will affect the range. Report follows once I have some time...