I realise that there have probably been countless previous threads on this topic but I wasn't able to find much information on my specific case.

I have 3 stick packs: Xcell 4300mAh 7,2V.

They were used in a 1:10 buggy with a 19T brushed motor and did work just fine (never got much warmer than 40°C after being used in it).
The charger always reported 4200-4400mAh discharge capacity.
Then they went into storrage for two years, probably completely charged.

The last two weeks the charger was reviving them at 500mA (stops when the capacity stays the same between cycles, but does at least 5).
One got back to 3800mAh and drives just fine but gets really warm, probably due to internal resistance having risen.
The other two keep 2400mAh but can only deliver very little current. Not even enough to move the rc car.

So what should I do with them? I did order two replacement Melasta 5000mAh packs (I expect them to do 4200mAh but that would be fine) but they will take a month to arrive.

I have read of "zapping" the (bad/highes resistance) cells with a car battery charger at 12V/20A.
Doing that on our terrace from inside the house wouldn't be a problem, no need to explode right in my face
But that apparently isn't a permanent solution.
Also, maybe just use the "good" cells to make two 7.2V packs, but how do I go about that?
How do I package them up again? Is there anything special about soldering batteries?
Should I discharge the packs before disassembling them?
Will I be able to tell the bad cells by resistance (-> multimeter) if I discharge them?

Or is there something entirely else I should do with them?