Thanks a lot @guido.gunther, helping me to understand proper approach!
And, to me now looks like that something like DUX DUCIS® PD3.0 (including PD PPS) car charger (over USB-C to USB-C cable) should be compatible with the Librem 5 battery expectations from its PD port. Same product (relatively cheap) Aliexpress links are here and here.
Note: This car charger isn’t tested from my side (no promise it will work as it should).