Great point about the location of the box not needing to be precise; in fact, it needs to be over-sized and thus imprecise, so that the light itself can be placed precisely at the end of the day. Keep up the great educational videos!
Another tip for locating your lights even if their boxes and wiring have been covered by drywall is to layout the room on the subfloor. For instance, the location of the kitchen counters and dining room table and where to put the spot lights for best illumination. You can then project these points from the subfloor to the ceiling with a laser.
For most 6" wafer lights, a 6 1/4" hole saw is the recommended size, 4-1/4" for 4" lights. Using a tool that can be dialed in to exactly the amount needed is better, but hole saws are quick and simple.
We used to use a hole saw. But we've found that each brand is a little different, and we've even found a few 6" lights that were closer to 5". That was a nightmare.
Some of them have a thin gasket sort of O-ring style around the edge of the wafer light. The springs on the little "holders" press the wafer light up against the drywall, compressing the gasket which air seals it. Also: why not just big stretch the rim of the wafer light? These things last for like a decade, no real reason to take them down frequently. A bit of caulk cleanup once a decade seems fine.
I have already replaced several within the last few years, I sure wish every single one of them lasted decades. That "gasket" is definitely not a proper air seal, especially on a textured ceiling. If your relying on gaskets like that throughout the house, your losing a lot of conditioned air. Perform a blower door test without an air sealed box above a wafer light and you will understand.
I admire the attention to detail, but you spent as much money (labor/material) making your enclosure as you did on the lights. Do your clients a solid and upgrade your lighting - at a minimum install regressed can lights to reduce glare. Lastly, instead of using OSB/caulk, just cut a piece of air barrier membrane and tape it down.
