Soldering a thin spring steel wire to a PCB
We solder stainless steel from time to time.  It needs more heat than I'm really happy bringing to a PCB, and the flux (we use a HCl flux, as you say) will quickly eat the tracks off the pcb if you leave any residue.  Our process is to tin the steel with the acid flux, then clean very thoroughly, then solder to the PCB in a separate operation. If you can buy the wire pre-tinned, all the better.

We can't use conductive epoxy for this for various reasons. If we could, we might consider it. But in my experience it comes a poor third place for mechanical strength, and good epoxies (i.e. Epotek H20E, not "silver paint for PCB repair") can be expensive and a little faffy to work with.

Another approach we've used is to crimp a ferrule to the end of the steel wire, then solder that to the PCB. That avoids the nasty flux, and gives a mechanically very satisfactory join. But it does take up a little more space.