Much depends on what kind of product or output you are trying to accomplish. The textbook answers are rolling-wave planning, prioritisation, iterative/incremental delivery, and, if the product is software, a DevOps style approach to delivery.
Don't feel constrained by project-thinking and predictive planning. Manage the product or value-stream, never the project. To see the difference, just consider that the kind of continuous improvement you described is actually normal and is often the desirable state in some fields. Millions of technology professionals build, develop, fix and improve software products continuously, and most of the time they can do that work without planning anything called a "project". What matters is to deliver frequent improvements that are of real business value.
If too much project planning is getting in the way then focus on working out what can be done this week or this month, start delivering and just accept that longer term plans will adapt and change as you go. That may not be easily achieved for every kind of activity, but it is very often the case that complex challenges are best tackled by breaking them down into smaller problems that can be delivered piece by piece.