From your description, my view is limited, there is no correct solution. Any choice is viable and fine, and any decision you make will be due to the reasons you chose with.

You didn't disclose what the alternative opportunity and field is, and also not your view on the field and you in it. So it's difficult to assess and put into relation.

You didn't disclose what you did before work, but two years is not that much experience for an engineer. Especially if it is not a particularly nourishing environment. You gain such expertise through experience and exposure over time. Depending on the project and environment it's also not enough to fully understand and intuitively know a big project.

At my workplace we separate role from [personal] development level. As a developer one's role may be developer or lead developer. The development stages are Trainee, Junior, Professional, Senior. If you can work on tasks mostly self-reliant (asking and collaborating is still open of course; knowing when to ask is a skill too) and can put tasks and work into context, you are a Professional. A Senior can support and guide the team. It is perfectly fine to settle for Professional.

Not being exceptional is not a good reason to quit. If you work and bring value, that's still value. Don't decide whether you are valuable or good enough for others. (This leaves out the question of what it means for yourself of course. Tackle those questions individually.)

You say you get your work done. Continuing to do that at a Professional Developer rather than Senior level is fine. You still bring value.

I want to know if that’s what it sounds like to people who’ve seen that before. If you were in my position, would you walk away and just be a hobbyist programmer or stick it out and hope to be a mediocre engineer one day?

I really can't answer that specifically.

You said your team environment is not the best. I assume you don't do retrospectives or personal feedback. Is feedback something you could ask [of some of] your team members, lead, or seniors? (Take care not to poison your question for open feedback with your negative assumptions of yourself and your work.)

Where would you like to be? Separating what you think is expected of you from your expectation and view of yourself and from what you enjoyed and where you think you would feel comfortable settling, how would you lay those out?

Have you considered switching project or employer? You have only seen and experienced that place. A different work environment could be very different - even in the same field.