I know what seafoam is. Putting it into your gas tank is useless because you can always clean out your fuel injectors with a full throttle pull to redline (if you redline your car once a week, your injectors and fuel system doesn’t need anything else)
Putting it into the crankcase is totally unnecessary unless you/the previous owner has been neglecting oil changes or your car has very high mileage. It’s actually quite bad for the metals inside your engine to use seafoam regularly, especially if you don’t drain the oil and change it right away.
Putting it into a vacuum line (for the intake to suck it up and clean your cylinder head, intake manifold etc. Is also totally unnecessary because your v6 is a relatively modern design with port injection which cleans the valves/ports of any carbon buildup due to the detergent-like nature of the gasoline. What I’m trying to say is that the seafoam is totally 100% unnecessary for you and your car, I bet you it’s doing more harm than good if anything.
Unless your owners manual specified that 91 octane fuel is okay, then 87 octane is totally 100% sufficient especially since that is what Mazda recommends for your car anyway. If you’re going to go out your way or pay extra for a certain type of fuel, I highly recommend looking for a fuel that has no ethanol in it, or at least 5% ethanol like shell 89 vs. The typical 10% blend you see everywhere else. You will notice better power and considerably improved fuel efficiency by switching to an ethanol-free fuel.