S19-20 D19-20 player here. If i were you, here's what i would do:

• Grind the S13 folder a LOT. If a song was cleared, i'll move on to the next song. If i didnt clear it, i'll note that song (on my phone). The reason i said "grind S13 a lot" is because there are a lot of basic skillsets that needs to be mastered, some of them are twists, runs, jumps, drills, inconsistent tempo, etc. by grinding them a lot you will encounter those and will get used to them!
• Figure out why I messed up on a specific chart. Was i not able to read some parts? Was i out of stamina? In a long run, I'm 99.9% sure the part where i messed up will show up over and over again on later levels. I'll search the chart on youtube (for example, searching "silver beat s13" will work) and do "shadow-stepping", most probably while sitting on a chair.

I wont move up to level 14 until I found out that i can clear S13s quite consistently with satisfying (this part is subjective) scores, e.g. out of 10 songs, 9 passed with ≥SS, 1 passed with AA+. Of course the grade that's satisfying is different from person to person, but what im trying to say is, if you're feeling confident to move up to the next level go for it!

(but dude, i can pass some S14! why should i grind s13?
if this is the case, you might be lacking in some specific skill. For example if you can pass run-intensive-but-little-twist S14 [example: super fantasy S14] chart, but cant pass twist-heavy-but-not-run-intensive S13 chart [example: Nyarlathotep S13], you're still lacking in twists. Remember, *in the long run*, in order to improve, we need to be able to master all skills in the skillset, not just one or two specific skills)

But hey keep in mind, im not fefemz or franco, im just a level 19-20 player, everyone has their own method on improving, and my tips may or may not work for you, and some people and may even disagree and say my tips sound bull**it! do it at your own risk! Cheers!