I can also add something to this but I can't say for 100%.
I built a testing rig to measure charge and discharge of the used 12V EFB+ battery, supposedly 72Ah when new.
I am a bit surprised and for this reason doubtful but seems like the battery only has about 25 Ah capacity left, so about 35%, while the on board system was showing as it having 83% of capacity left - usable battery capacity when full.
Sadly I do not have a brand new battery to test this out and see how much charge I can put in and take out, so take this with a big amount of salt, but basically I did this 2 or 3 times now, measuring how much energy in Wh and Ah I put in and get out and it seems to be at the 20 - 25Ah range.
In the screenshot you can see all the curves, altough because of some error the top left is marked "Wh", it actually represents the voltage, I current in amps, and below power and amper hours, This is the discharege of a fully charged battery.
And here are the charging charts of a fully discharged battery (around 11.6V empty)
For anyone wondering - this was done with INA211current sensor and ESP32 microcontroler, the dummy load was a heater element.
I also did another test where I connected the used battery to a 12V to 230V inverter, connected a handheld vaccum machine to it, and it was able to put out about 500W, powering the vaccum, while the voltage on the battery dropped to 10.8V, so about 2V, if I can trust the voltage indicator on the inverter. Using a much smaller 12V battery, that with my tests puts out about 12Ah of capacity, the vacuum would not even start because the voltage drop on a much smaller (not a car lead acid battery) battery is so high, the inverter doesn't work.
I am planing to use this used battery with an inverter for camping reasons or for a portable 12V refrigerator I have, should be able to power it for like 2-3 hours.