> GPS is also unreliable in Ukraine, especially near the front line.

There are GPS antennas that physically can block out signals that are not coming from the sky with a huge amount of decibels. Maybe Aliexpress has some of them in stock? This was heavily ITAR-ed but this ban was lifted recently.

Other option: try to sync against the DCF77 signal from Germany. Not only the beep-beep-beep time signal but also the integrated phase modulation. Jamming VLF is difficult. 77KHz is in the range of ADCs.

Then make a voter: if GPS/GLONASS/Galileo/Beidou is available prefer them, if not fall-back to DCF77. If this fails: free-running.