1. Merging often and early might be viable, but it might also expose your colleagues and the codebase to your exploratory process, which might or might not be desired/possible. If you're making semi-permanent changes like API modifications or database table updates, going this route will mean a lot of extra work.
2. Too many chefs. If you open up for too many questions at the start, you will get a gazillion conflicting opinions thrown at you. You'll need to limit the audience somehow, or bring forward a clear idea or opinion. This leads to developers working a bit on the side. I don't mind this, I think it's good to give autonomy to working groups and let them make mistakes, as long as there is coaching or feedback in place in the long run.
3. If you go the route of designing a bit on the side first, to save time compared to 1), you might suddenly find yourself in the situation that the major parts feel quite done. Adding polish is now just a minor addition to what you're already accomplished, so you might do that before putting up the PR. This means that a complete solution showing up as a PR doesn't necessarily mean that it was over-engineered or overly polished on the side.
4. Not "syncing" too often saves time; less context switching, distracting information, and time to reflect on decisions and design.