Great developers are overrated. 99.9% of software needs competent developers, even if great developers are/can be a plus. Greenfield projects are also the least professionally stable, usually start-ups, and the majority of jobs are centred on maintenance and aditionality.

So for me, your issue is not that you're not a great developer, but that you're tired, and you're going through a shift in your professional identity. It may be that the aspiration of being a "great developer" may have been a big part of your professional motivation, which I think is more common in early to mid-career, and your feeling you are not is demotivating. Your sense you're not a great developer is of course itself not super reliable. When my grandfather was greeted "How are you?", he would answer "compared to whom?". I remember a fantastic, internationally respected developer, generally very confident, feeling very nervous about giving a talk around peers he felt were (even more) expert than himself. Your sense of mediocrity may in fact be an indication of excellence and experience, midway along the dunning-kruger curve. Your standards of greatness have risen and your version of mediocrity has widened.

Which is not to say you're wrong. By your standards, you may have worked with colleagues you consider "great developers" and your judgement of being middling by comparison may be accurate. But there's nothing intrinsically wrong with that. You're evidently not a bad developer, just not as "great" as you'd like to be or thought you'd be earlier in your journey. The question is how much weight you choose to give this. And it is to a large extent a choice. Probably the vast majority of devs are happy being competent, and don't feel they are great, and it is not crushing. They get their job and life satisfaction from a job well done (greatness or not), a good team, a good product, a good salary, and life outside their job.

I was curious that you said you don't value your work on greenfield products but would like to start your own thing, presumably a greenfield product. That reinforces my sense that you're fully competent, including in greenfield projects, even if you don't feel you're great, whatever that means to you. My advice would be, since you also value stability, to stay in a solid maintenance job, and in your spare time join some motivated non-technical co-founder (there are networks for that), and together find an idea you can work on. If it takes off, you can take the leap, and if not, you can scratch the itch and learn from the attempt. Meanwhile, I would encourage to reframe your professional identity and motivation and satisfaction away from the inherently socially comparative "great developer" ideal, which, however great you could actually be, would be psychologically precarious, as you would merely elevate your comparison benchmark so impostor syndrome would not necessarily vanish. Find satisfaction in your work, your progress, your growth, your impact and your environment, regardless of where it ranks against whoever your are currently comparing yourself to, explicitly or implicitly. That is something that will benefit you wherever you move toward next, and not doing so will follow you wherever you change to.