I have a Lifetimer I had built in 2006, and I am on my second nylon plug. It comes out every time the boat comes off the ramp, that is likely in the hundreds. I tighten it and remove it with the screwdriver attachment on my Swiss Army knife, so it is never that tight. It has never leaked, or given me a bit of trouble.
The drain is just a half inch aluminum conduit coupling that they weld into a hole they bored through the transom. I asked them to weld on a half of a coupling high up on the transom to hold the plug, so I remembered before launching, and didn’t have to search for the plug. So far, so good. I have yet to launch without a plug installed, but who knows, life is not over yet.
I think the key is not trying to insert something harder or incompatible with the aluminum. There is no need to force the treading. If it leaked a few drops an hour, does it matter? Nylon in to aluminum is great. Should last forever without an intervention.