welcome to the site. So I gotta ask, do you really pass by the Oakville plant yet live in Sault Ste Marie?? Isn't that about 800 km whether you go thru the states or stay in Ontario?
On a mechanical note, I must say I haven't done any of the procedures that you mention but I suppose if they're working for you, keep it up. I've heard some horror stories of trans and engine "cleaning" only to have failures after the cleaning. I guess if you start with that procedure early and the motor is "used" to it things would/could be ok.
I'm interested about your findings regarding the spark plugs. I've found that the OEM are the only way to go and have heard confirmed here basically the same. The "replacement" or "identical" plugs by other manufacturers tend to cause problems with ping and codes etc. Glad again that is not your finding.
If you want to add to your preventative measures, you could send an oil sample to Blackstone Labs every once in a while. I do with both the trans fluid and oil about once a year. It has revealed a lot over the years on the 3 vehicles I maintain. Cheap cost for lots of valuable data. Like running bloodwork on your car. (for what it's worth, I have no interest in the company...)
I too have a 2000 3.8 and am working hard to keep it on the road for another 3 years or so. Wish me, and all others, luck!