Another problem I see is that I can't do this with sacrifice branches, like the long one thickening the base of the trunk. If this branch gets too thick it will be a problem. Should I cut it before it gets too thick?. Thank you for your time.
It looks like you've got a nub at the base, just near that sacrificial. I'd cut that before I cut the live one. The proximity of the live one may help the whatever callus/healing process you may get out of it. I still wouldn't leave it on too long; it's about as thick as I'd go. I'd cut it by end of season.
Since you've been doing research on it, I'm sure you're aware: forsythia sends out suckers & buds no problem. They're just prolific around my area. You'll probably have several shoots over the coming years that will help thicken the base up. Let them grow for a season or two, then cut back.
You've already got several around the upper right knob, especially in the crotch area. I'd pinch several of those out or you'll have a big reverse taper there.