Should i be worried about ecoboom or using high quality gas?
No. If you are tuned, then I would definitely recommend running high octane gas. But if not, there are plenty of safety features on the stock tune to keep things in check even when you're really laying into the throttle and are on 87 octane for gas. You're just not gonna get the "full performance" out of the engine. But to many people, that's all good.
Overall I'm just trying to figure out what I can do other than basic maintenance (coolant, oil, spark plugs) to get the most mileage out of the vehicle.
Fully synthetic GDI formulated oil. GDI engines tend to run dirty. You want use oil formulated for it. The "synthetic blend" that the manual calls for is a one way ticket to sludging up your engine and intake valves as quickly as possible.
You're good on stock plugs. Just check your gaps every once in a while and replace no more than every 3 years. Some people, like me, replace every 2 years.
And like I said earlier, get yourself an aftermarket intercooler and replace that piece of garbage on your nose. The best you can do to keep intake temps down, the better your turbo car will run.
This being said, is looking to get 200k + miles out of my 2018 Mustang Ecoboost Convertiable possible?
I'm at 78K right now. And the engine still purrs. Can still score a low 0-60. I feel like there will be 'other' possible gremlins that get me off this car, before the engine or tranny itself (AC, electrical, etc). If I can avoid those gremlins, I may try to take this car to 150K miles.