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I would use your VNA to measure both antennae * cables for s11,s22 and s21 at 1m gap.  Set the Tx to a suitable level and wide video bandwidth.

Then use the Tx antenna on a LORA and attempt to capture the peak pulse power  on any of the chirps for possible correlation from the same distance and use Friis loss attempt to estimate Tx levels. Similarily for RSSI from VNA Tx.  Room reflections may interfere but don't risk VNA Rx overload with direct connections.  Capturing the sweep rates of both LORA and the VNA will lead to random levels but using the peaks may be useful.  Your Tx BW and sweep rates will also have some effect on RSSI threshold but not RSSI levels at a fixed distance.

What else can reduce your excessive path losss shown by the RSSI?

Can you scope Vdd ripple with a diff probe or tip/ring probe?  Could this cause VCO Tx dither and reduce RSSI?  PC audio scope may work..

Use a commercial LORA card & Antenna to compare results.

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