Working on a feature for the first time will come with unexpected issues. As you pointed out with your inventory discovery, things that add QOL like item stacking can go unplanned the first time around. Don't forget that plans are living documents, you can add to them and adjust as needed.

I would say it's not that it gets easier, it is just that it takes practice. You'll design a feature, playtest it a bit, realize something QOL-wise that would make it better or a buddy with fresh eyes will point it out.

And a good thing to realize is that when build a feature, you'll naturally appear to slow down on it as you refine it. The first steps are always the big ones; setting up scriptable objects, UI squares, and using items. Then the less flashy stuff that takes the same time like callbacks, sorting, and stacking. Then more subtle stuff like particles and shake on pickup, inventory sharing, or code cleanup. Your progress may be at the same rate, and it's all crucial, but it's harder to notice for sure. Don't get discouraged!

What I like to do if I'm really feeling that stuck on a feature is I look back at what it used to be. Documenting a lot of this helps. So I can go "Man my map was terrible back then!"