Another board with the very last Si4421 in the world was fabricated. This chip was a real buster. It wouldn't start up reliably. Clever switching of Vdd to 0, then 3.3V got it to start up but when it did, it was transmitting way up on 1087Mhz & sucking a lot more than 50mA. Changing the frequency register didn't do anything.

Barring a layout error, the very last Si4421 in the world was dead.  There were a few routing errors manely involving GND, but the power supply didn't allow it to power up.

It actually drifted as the crystal temperature changed.  Higher temperature pushed the frequency down.

915Mhz wasn't very appealing because at the weather station's 8192 baud rate, it was going to be on 100% of the time, jamming the weather stations. It would entail changing everything to 100kbaud.

With 3 dead radios, the lion kingdom seems to have run into the limits of the spare parts. Another idea began to hatch, involving running the hiletgo on 12V for better range & transmitting to the router to avoid motor noise. The market seems to have shifted to low cost AM radios for instant startup, hence the lack of any more FM radios with instant startup.