I don’t know how many terminations of this type you might have performed however I can say with confidence and from experience that to use a seperate flux will provide no benefit here. It certainly won’t be of any use in removing the polymer insulation as you have suggested it might.Forget seperate flux- we aren’t doing plumbing. Flux cored solder will work better and is easier to work with.
Personal preference.
There are differences between the techniques used by plumbers and electricians. I was hoping to not complicate the task, as I think the op has his hands full.
As for working better with flux core type solder in this instance, I think not.