thank you very much for your answers!

so I hooked up a car brake light bulb to the batteries and measured the voltage of the individual cells (sewing needles though the heatshrink tube) under the 2.5A load.
Some of the cells keep delivering 1.15V just fine, others went as low as 0.8V under load. With the packs coming fresh of the charger.
So right now, I hooked my chargers up to the weakest cell on each battery and let them do a "revive".
We'll see if that works... at least for me it has in the past with other seemingly dead cells being back to almost full capacity after a few days.
But you are right, current LiPo tech is much better (capacity per weight, maintenance is automatic, no need to discharge first) but also a bit more expensive for the same capacity and I'd have to get a LiPo charger first.