Sounds familiar to some older campaigns ive done.
The downsides are that as well. Players dont care to much about town of the week and BPCs of the week.
Very similar structure to the old Savage Tide AP. Are you familiar with Savage Tide?
I played it a loooong time ago, yeah.
So here's my big thought for the season arc/metaplot for an adventure so that it has some attainable goals and meaningful impact without 'destroying' the setting:
Prologue is the invasion. The war has begun, and your party is fighting back against it.
Act 1: You're the new Thundercats/Masters of the Universe/Silverhawks heroes of light/eternia/whatever. You're tasked with, essentially, guerilla warfare against the alien invaders. Fighting asymmetrically against their forces by targeting specific engagements to slow down their attacks. You go from place to place meeting new allies and old enemies and work towards being the big heroes. You face some setbacks but you're mostly successful. The big mid-season event is that you work with your allies to break some kind of super powered relay station to disable the Jump Gates/Wormholes/Portals/Whatever so the aliens don't have infinite reinforcements coming from countless other worlds.
Act 2: The aliens double-down on their attack. Cut off from reinforcements they instead try to make a decapitating strike against the leaders of the world including your party. Out come the Siege Engine Monsters and the Wrought Dragons and stuff to try and destroy you, the bad guys you've been allying with against the aliens, everything. Friends you made in Act 1 are in peril and might get axed if you don't work to save them in various new locations 'cause they've been helping you slow down the aliens. Malevar gets involved, -directly-, and gets disintegrated at least once, possibly saving a party member. He laughs as he's destroyed because he is -Eternal-.
Act 3: The final push against the aliens to take out their leadership and/or their engineering teams who are trying to get the uplink online so they can, once again, have infinite reinforcements. Malevar returns and helps to fight in order to secure more power for himself. After the big knock down drag-out with the aliens, you have to fight Malevar directly, as he once again tries to consume an energy source bigger than his inflated ego. Just like 800 years ago, the Heroes of Light kick his butt back to his castle to sleep it off and plot for his next campaign of evil.
Thoughts on the overarching structure?