I used to be a planner, but TBH I found my outfits became sort of boring. I think I planned as a coping mechanism to anxiety and stress (infant at home, long commute, stressful job, no energy for fun outfit enjoyment at 6 am in the dark after 4 hours of sleep). Working from home has taken that away and I've become more spontaneous.
I do still plan when I feel I need to shop or purge.
If I find a piece I love, I will order it (between work/kids/pandemic, in person shopping is completely nonexistent) make sure it fits and is the expected color/fabrication, then lay it out with a few items to see if I can make some outfits I like.
Conversely, if I haven't worn an item in a while, I'll do the same thing to determine if I still love it.
I also keep a notepad with some outfit ideas - sometimes more baked than others - so if my creativity runs dry in the moment I can refer to it for some fresh ideas. For example, I've got some Fall/Winter ruminations around waist-length woolen jackets, flat boots, and pleated printed or colorful skirts.
Sometimes I keep coming back to a certain item and various styling ideas. So I keep my eye out for that item. Most recently I acquired a satin bias-cut knee length skirt that I've had some ideas for for quite a few months (deep sale, non-returnable, thank goodness it works!) It will slot in nicely to the outfit ideas I have. But I keep it loose and don't assign specific outfits to days because I've found that half the time I'm not in the mood for what I planned and then it feels like a waste.
That said, playing with styling ideas is totally my jam and feeds my in the moment creative process.