Can I use WD40 and generic bicycle oil to maintain my MTB drivetrain?
You're basically using WD40 as degreaser (it's a very thin oil that will dissolve away thicker oily gunk) and then using chain lube as chain lube. This is fine, as shown by the fact that you've been doing this for ages and not had any problems.

The only thing I'd suggest is that there's no need to use "copious amounts" of chain lube. It doesn't take long at all to just apply a dot of lube to each link of the chain as thown in the video, then let it soak in for a  couple of minutes and wipe off the excess.  Alternatively, hold the nozzle of the lube bottle against the chain at one end of the rollers and use the pedals to rotate the chain once round, while dispensing a thin stream of oil, then repeat for the other end of the rollers. If your chain has a quick link or similar connector, you can use that to see when you've been round once. Ultimately, though, since you're wiping off the excess, the only question is whether you prefer to use less lube or less of your precious time.