Is the check plug really even necessary for a trans fluid swap a lot of the time?
I just drain mines and refill to specified amount, I've never once cared for a check plug and just change it every ~50k miles on most of my cars, regardless of color. It's usually not an expensive fluid.
On most cars you simply fill till it starts leaking out of the fill hole - like changing oil, it normally doesn't require a precision fill, just close enough.
1. The drain hole is below the fill hole - open drain hole and empty using good old fashioned gravity.
2. Close drain hole.
3. Open fill hole.
4. Fill till it starts leaking out the fill hole.
5. Close the fill hole.
Not especially complex most of the time! Transmission fluid does not really shrink much over time, its a sealed system and it doesn't burn off the way engine oil can. Just doing it at regular intervals like 50k miles is absolutely fine on a lot of cars. If you really do care about hitting some specific quantity of fluid - you know exactly how much you are pouring into the fill hole, you can even measure exactly what came out the drain hole, but this probably is not remotely necessary.
On the E36, which has no prescribed transmission service interval or dip stick, there are two transmission sumps -- and only one of them has a drain plug. Replacing the fluid means pulling one sump off completely...and pre-filling it prior to reinstallation.
On the Honda, which has a regular interval for transmission fluids and a dip stick, the fill plug is on top of the transmission. It would be a bad thing to fill it to that level. (I just refill using the dip stick tube.)
Also, I'd like to add that I am envious of the fact that you have never experienced a leak in any automatic transmission.
I just drain mines and refill to specified amount, I've never once cared for a check plug and just change it every ~50k miles on most of my cars, regardless of color. It's usually not an expensive fluid.