The Red Line fuel system cleaner is good stuff if your engine needs it. The only way to know if it needs it is to spend the $12 and use a bottle. Can't hurt (except the $12) and might be a little help.
Changed my plugs at 41k. They look pretty good to me to be changed so early. Very simple DIY only need to remove 4 bolts (one 10mm, three 8mm) to have enough room to remove the coils and get to the spark plugs. Next maintenance project will be the throttle body!
Go to the dealership only for warranty repairs or repairs too complex for an independent shop. Any shop can do spark plugs, and it'll be much lower cost than the dealer. You know the dealership will also try to scare you into additional repairs as well.
40k would also be a good time to drain & refill the automatic transmission for owners who expect to have their car for a few hundreds of thousands of miles. Modern ATF is probably good for 100k. Draining and refilling changes something like 40% of the fluid, so doing it periodically will keep the fluid fresh and protective.
2nd Generation Mazda CX-9 (2016 - Present) with SKYACTIV TECHNOLOGY will have lifetime transmission fluid that only requires changing should a critical component or the whole transmission need to be replaced. Spark Plugs needs to be changed every 40K miles to avoid misfires because the engine is turbocharged and has a high compression ratio.
Changing spark plugs @ 40K miles are pretty common for turbocharged direct injection engines to avoid misfires especially the SKYACTIV TECHNOLOGY we have in our CX-9 which have a very high compression rate for a turbocharged engine.
2nd Generation Mazda CX-9 (2016 - Present) with SKYACTIV TECHNOLOGY will have lifetime transmission fluid that only requires changing should a critical component or the whole transmission need to be replaced.