I’ve printed a number of print-in-place items, and designed a couple. For your application, there are two separate issues that you need to solve. First how to stack the nut so that it breaks away from the bolt, and second, the correct scaling/spacing for the nut.
To get the bolt to break away you will first need to support the nut. You could do that by increasing the size of the top flange and having the top down on the print bed, or by redesigning the bottom flange. I’ve never designed a “stacked” breakaway. From what I’ve gleamed from others, there are two things that must be considered. First, there needs to be the right amount of separation between the two items. In this diagram, I’ve used 0.1mm, but I have no idea if this is the right amount. It should be quick to run some tests. The second issue is the height of the separation. If I remember correctly, you want the separation to start on a multiple of the layer height you are using for your 3D print.
As for sizing the threads, when I 3D print threaded items, I either increase the size (XY only) of the outer piece (nut) by 0.2mm or decrease the inner (bolt) portion by 0.2mm. For my 3D printer, this works pretty universally no matter the thread size. For print in place for a 30mm bolt, I probably start by doubling this number to 0.4mm.
Fusion scales by fractions, so assuming the nut is scaled, and using the 0.4mm, I get 30.4 / 30 = 1.0133.
So, with the nut in the orientation of the diagram, I would scale X and Y but not Z by 1.0133.
Edit: I don’t know what the requirements are for this 3D printed part, but it occurred to me that you could chamfer the bottom of the nut thereby 1) making less contact area so the part would be easier to break free, and 2) reduce the amount of the nut that needed to be supported. As long as the chamfer was less than overhang your printer/plastic would handle well, it should work (say 50 degrees).