I have soldered for 55+years, a large part of that as a professional consumer electronics tech,
and I never really 'needed' flux in all those years.
I do have it in stock for difficult jobs, but it's usually more of a hassle than a cure.
Learn to solder properly, with a temp controlled iron, and you don't need it.
I frequently hand-solder dozens of 0.5mm pitch PIC chips with just 0.7mm rosincore solder.
Looks no different than reflowing if you do it right.
Leo..