I’ve spent many hours trying to recover numerous lead acid batteries over the years, using various methods like salts and electronic high-frequency “desulphators”. Sometimes you can extend the life by a year or two, but I’m not sure it’s worth the time invested.
Most lead-acid batteries die prematurely through underuse... being left in an undercharged state for a period of time where sulphate forms and hardens. Once that happens, you’re flogging a dead horse. An aged battery can also suffer from shedding or other plate defects – again, it’s irretrievable.
You also need a capacity checking method/device to test the cold cranking amps to see if a battery is “repaired”... at least if you want to rely on it for another year or two.
My own belief now having wasted many hours "messing about" is simply bite the bullet, buy a new battery and then look after it (like you would your e-bike battery). If a car is going to sit idle for more than a few days, keep a “smart” battery charger connected to keep the battery topped-up and maintained to prevent formation of sulphate. These cost as little as £15 from Aldi/Lidl up to £60-£200 for a premium brand like Ctek. These will extend the life of your battery, so I think a very good investment.