How would I go about making a GTA V style weapon wheel?
The question is a bit too broad to answer it straight.
I’d start by making each of the wheel slices a separate widget so you can reuse the functionality. Treat it as a weapon slot and populate it with a weapon (provide the slot with a weapon object reference, for example); manage empty slot behaviour here, ammo counters, highlights and so on. An overlay as a slot + image and a text block should be enough to start with.
Use a separate widget with a canvas to assemble it in the Designer and handle the rest of the required functionality here. You should have direct child widget references available at this point.
The one major issue you may run into is the shape of slice and how cursor interaction works. The UMG does not support pixel perfect collisions out of the box, unless something changed recently. You can create a workaround solution by using approximation and stacking borders in a vertical box to form a rough trapezoid shape - it’s usually enough for the players to not notice the slight shape inconsistency. Otherwise you’d need to make your own custom solution or use someone else’s but its integration may complicate things a bit.