Undiagnosed battery problems, not only fault codes but a safety issue

I have battery problems on my MK2 Octavia, I bought a new Varta E44 a year ago as a precautionary measure but it always had a low cell voltage from day 1 plus my vehicle has I think a slightly excessive quiescent current drain, trouble is the PD engine always starts on 1/4 turn so you dont even feel that the cranking is weak.

Then one day I tried to start it and it wouldn't, I hadn't driven it for a week or so and in doing so further damaged the battery, as expected it generated loads of fault codes all low voltage and canbus comms related and was fine once they were removed.

I fitted an LED battery readout and try not to crank it unless its has 12v and only then after the glowplugs have stopped doing their thing and the voltage come back up again albeit sometimes to 11.8v.

Once I forgot and it brought up the ABS traction control and ESP lights, they went out after a hot restarting later in the day, VCDS logged them as canbus comm errors with the ABS module amongst others.

Today I started it from cold after the gloplugs had operated and the voltage raised, no problems, drove about 5 miles and restarted 10 minutes later without waiting, temp guage was half way up, warning tone and the 3 warning lights again as before, would not go out on restart but did later in the day, on the drive back I wondered if the systems were working while the lights were up so hit the brakes on a safe straight bit of road with no other traffic, no ABS 🙁 so I assume that the ESP and TCS would not work either, was not going to test them.

I think that Wino has helped me identify the parasitic current drain, I had already fitted a later canbus gateway because of a head unit incompatibility, the battery will be changed in due course and I will be very vigilant with my start procedures, its scary that a slightly discharged weak battery that the seller will probably say is fine and which does indeed start the car can create such a serious safety issue, I have seen many cars that drive around for a couple of years between CT tests in France with the dashboard lit up like a Xmas tree, until today I thought that the systems would function again if the comms were restored with the raised battery voltage from the charging system and that the lights remained on until the fault codes were read and reset like the airbag warning system but no, it would appear that they are inoperative.

All the electronic circuits that I ever designed and made for vehicles used 5 volt logic chips, the main ECU will operate even when the battery will no longer crank the vehicle, probably down to around 9 volts, why do the canbus and the safety critical systems fall over during normal cranking voltages when the engine will start easily?

Are the later generation cars like my Yeti just as flaky? What about other manufacturers? somehow I cant see Honda, Nissan Kia or Hyundai accepting that.