Trying to screw together two halves of a print, and struggling

I’m printing the Umikot wdt from https://www.printables.com/model/481587-umikot-planetary-gear-spirograph-espresso-wdt-tool

And all the pieces seem fine, but I can’t seem to screw together the base and upper mech fully.

Initially I could get it one full turn before it got caught on something. With force, I managed to get it almost fully screwed on but not quite, it feels like there just isn’t enough tolerance.

I’m tempted to either try printing the base a small bit larger to allow for tolerance, or maybe just combining both parts into one and reprinting it in a single go. I do see a bit of z banding which I suspected might be the problem, but trying to sand at it barely helped.

Any tips for this? Thanks!