I’ve just been getting into the more involved half of the hobby recently (this is the second kit I’m painting), having done straight builds on a bunch of stuff before now. My tendency so far has been to de-nub, clean, and assemble the kit first, figure out if I want to go all-in on painting it, then disassemble it for painting. This isn’t without its issues, you have to be pretty damn careful getting some of these bits apart or you’ll be left with an annoying repair job while you cry over your pile of broken plastic. In the future, I definitely want to paint before building, but I suspect that my enthusiasm for the construction process will get the better of me again at some point.
To actually answer your question, I tend to put parts on sticks in chunks that can reasonably be painted from all angles - in the case of that leg, it was the upper and lower ball sections, the foot, the lower leg front, the thruster, the thruster shroud, the lower leg back, the weird channeled slidey bit just behind the knee, the upper leg minus inner piston, the inner piston, and the two halves of the calf that hold the upper leg to the lower leg. A lot of times, though, things aren’t that bad - I painted the arms on this as just the arm with hand attached (fingers spread), shoulder armor, and shoulder armor clip thingy. Mount that all on some bamboo skewers with alligator clips crimped onto the end (and occasionally some poster tac for small parts like thrusters and v-fins) by grabbing parts that won’t get seen after assembly, throw on some music, and go to town. If you want parts of the same object to be different colors, you have to spend time masking the areas with some masking tape (I quite like Tamiya’s stuff, it’s easy to work with and low-enough tackiness that it generally doesn’t peel off your paint along with it, though you can typically varnish between paint layers to reduce the chances of that happening anyhow. Metals are kind of a special case, because every layer on top of them reduces the shininess from the tiny particles in the paint that reflect light, so you get the fun of gambling with durability vs. looks.
I also tend to work in batches that let me streamline my workflow. Here, I just painted the inner frame of the thing because it was all metallics, all the time, and I wouldn’t have to switch colors mid-session much at all. I got to prime everything, wait 12 hours for it to cure, paint everything gloss black, wait another 12 hours, paint everything in duraluminium or chrome as appropriate, wait a few more hours, mask off the areas I needed to so I could get the gold on, spray the gold, then come back the next day and spray sealant on everything. When I get to painting the armor, I’m going to break it up into color blocks - I’ll paint everything that’s white at once, everything that’s purple at once, etc. That’ll let me mix large quantities of colors if needed and not have to worry about finding the appropriate color again in the future if I forget the recipe or whatever.
Airbrushing’s not particularly hard once you get your setup dialed in and figure out the right consistency for paints, though the latter is definitely a skill you have to learn and probably the third-most time-consuming part of airbrushing (#1 being masking and #2 being cleaning). As an aside, one of the nicer things about the Alclad paints I used on the frame here is that they don’t need thinning at all, which saves a whole load of time. Probably my biggest tip to people who are just getting started is to grab a bunch of cheap pipettes for transferring paint/cleaning fluid to your airbrush, along with one of those airbrush cleaning stations to spray into when you’re running pure thinner or whatever through it between colors/at the end of the day. The only other real hurdle I find I run into is troubleshooting clogs and stuck needles, but all of that just comes with practice, and is a lot easier to avoid if you’re a little compulsive about partially disassembling and rinsing out your brush at the end of a day’s work - just pull the needle and the nozzle, wipe the needle off (carefully, so as you don’t bend it) so there’s no crud stuck on it, and run some water (or solvent if there’s truly nasty buildup, just be careful with it because it can eat the rubber seals that keep the air and paint flowing correctly) through your brush for a bit, then let everything dry before you put it back together. I like using one of those little lunch item-sized reusable containers with a folded paper towel in the bottom as a place to let everything dry out without it going everywhere if I’m stupid and bump it for whatever reason.
Wow, that’s a lot of words. tl;dr - yeah, I break everything down into manageable pieces so I can paint all the visible surfaces before assembling. Also airbrushing isn’t hard, but also it can be a bunch of work? I’unno.