The GM tubes from Bulgary arrived today, to my big surprise - I didn't expect them this month :) It becomes more and more obvious that the major cause of all the shipment delays is the German import duty office.
I hooked the tubes to my sparrow nest HV supply and tested them immediatelly. Both work fine, the sensitivity is much, much higher than the SI-3BG. While my hottest uranium ore barely made the SI-BG3 tick, the new tubes go crazy. The tubes are SBM-20 and SBM-20U. After I ordered them, I found out those are basically the same spec tubes, just the U variant is specified for more humid environment (or something alike).
Anyway, the analog part of the GM counter works fine. I'll replicate it in more good-looking fashion and add the digital part to it. I am still waiting for some small inductors from China, the one I am using right now is rather bulky. But so are the capacitors (the circuit uses voltage doubler, which increases the amount of capacitors to three), so maybe I will just use what I have. The whole counter draws something like 0.5mA from 5V power supply - not bad!