You can get a fair bit out of Hickman's FF without reading the 570 issues before it, though it certainly does help. I would like to see what a new reader would make of it if they'd only read his run.
I've only read Hickman's run. Mostly it works, there's just a few things that are confusing (the whole city in which relative time passes much faster for exemple).
Galactus I think they outright tell us it's an alternate future one, so I didn't bat too much a lash at it.