You will need to invest in a battery tab spot welder to be able to build/repair battery packs. Soldering is not a good idea - it is both more likely to damage the cell (heat) and less durable for a tool that is going to regularly experience shocks and vibration. You do not want those solder joints to crack and start arcing and burning holes in your lithium cells, as we know…
Most DIY-level spot welders can only handle relatively thin tabs made of nickel strip (as opposed to the “proper” material of copper), usually 0.10mm and maxing out up to like 0.20mm thickness. You need an industrial machine to spot weld copper tabs of the thickness (0.50mm+) usually found in tool packs - both the copper material and the much higher thickness make it a lot more difficult.
You need those really thick copper tabs for the type of power a modern tool can draw. Not totally accurate numbers, but to illustrate it, 0.20 nickel flows like 5A and you need that 0.50 copperto safely handler the 50+A that tools can draw sometimes. I have a spot welder and I use it primarily to build e-bike battery packs where I have like 7 parallel banks of cells. In that scenario, the individual draw from each cell isn’t excessive and thin nickel strips are appropriate.
I HAVE used my spot welder to rebuild tool packs (and will continue to as needed), but I ONLY use those packs in very light duty applications. When I run a high drain tool, I use a healthy OEM pack.