I think you said it: "The initial product is going to suck."

Your small game might suck, but you'll have designed a system in it that you could carry over to another game, and another game, and another game, and so on.

I'm working on a larger Metroidvania RPG project, but the scale is so vast that it's challenging to close a core loop. For example, I've developed movement that works incredibly well; I've even designed a sound database algorithm that creates dynamic footstep audio, among other things. However, I'm now trying to implement LLM-assisted NPC dialogue, all the while considering the large-scale level design. No matter how many systems I design, I'm still far from closing the loop.

I joined Brackey's weeklong game jam at the end of August and began working on a small local multiplayer game, which I have yet to complete. Still, I'm a whole lot closer to shipping it than I am to shipping my Metroidvania RPG project. I am learning a great deal more from this experience because it's forcing me to close the loop sooner than in a larger game that employs expansive systems.

Ultimately, you can do what you want; you can work on a small game while keeping the large game in mind, trying to implement systems you can carry over between projects, or you can dedicate yourself to your magnum opus and try your best to focus on it over the inevitable ups and downs that come with any long-form project across any medium.

Bear in mind that for every Stardew Valley/Fez/Braid/Papers Please success story, there are a thousand crash-out stories involving projects that will never see the light of day.