All of my responses above apply to the lead free solder in my MachineDrum circa 2010. I would have stated otherwise if this was not the case.
I can’t recall the reflow temp when I worked on it. It was not easy and I experienced similar difficulties to what you described.
I am currently using lead free solder in a few projects and my iron is around 320 degrees celsius.
Various online sources state that reflow temp should be lower for lead free alloys.
“The higher melting points of the typical Pb-Free (Sn/Ag) solders requires reflow to occur above 240°C. The recommended soldering or reflow profile must assure a minimum time above 240°C”
Perhaps your iron does not have an accurate temperature readout ,or the heat transfer of your iron tip is not sufficient