As above, no extra flux needed for wiring if you use multicore solder. If it's not working for you, maybe the soldering iron isn't up to the job, or you aren't "tinning" the connectors/wires before making the joint?
Personally, I avoid lead-free solders. I don't think they flow as nicely as the old tin/lead solder. I buy my solder on eBay, as I don't think tin/lead solder is widely available in European retail outlets anymore.
I do use flux for larger jobs, like brass or steel parts. Flux on both parts, then "tin" with solder. Hold both parts together and apply the iron. Shouldn't need anymore solder adding at this point.
I've got a tub of "Fry Powerflow" paste flux that I've been using for well over 30 years.
Different matter for hard (silver) soldering. Flux is essential here, as the job will start oxidising before the solder melts.