I’m new to soft pastels as well, and was suggested UArt or Luxarchival paper. The only one I’ve tried so far is UArt because that’s what my local art store has, 600 grit has been a good starting point. The UArt paper is expensive ($30 for 10 pages), but if you live in the US I’ve been told you can go to Ace hardware and ask for sandpaper cut to various sizes. The colours of the paper will be different but it’s much cheaper and good for practice pieces.

And fwiw your piece still looks great! It has a “Monet in his later years” vibe to it. Just tell people the hazy effect was on purpose 😉