I see, so just major refactor, or almost live-rewrite of a very messy code base, but mostly in the medium/small design space. Ya that makes total sense. It's mostly we already did this and have figured out the ambiguities, we just need to clean it up to basic coding standard and code structure/patterns.
Indeed it's more like live-rewrite; features are still wanted but the rewrite heavily influences them as they go so it's juggling a lot of balls at the same time.
Fable 5 was amazing at helping me with that. Opus 4.8 is... good, really good, but... you have to be on your toes _all_ the time. It gets tiring. But as already explained: it also helped me become better.