I don't want to be rude yet the mapping is bad :
First of all, thanks for writting an honest review, it's the only way I can know which aspects have to improve. I have some questions about the things you highlighted tough, i'll do it in order:
Notes lacking is seen across all the beatmap or specific parts? I have to admit that parts like from 1:06 to 1:09 or 1:45 to 1:47 shouldn't be virtually empty as they're high parts of the song, but may I ask if it's something generalized?
2. Shape of the circles? You mean sliders? Flow is kinda bad to be honest, it's something i'm working on.
I don't know exactly what you mean with overlapped reverse sliders, too long maybe? Or you actually mean that they overlap? I'll fix it anyways.
I had an aneurysm trying to add hitsounds, so it's something I will do lastly.
Is this in reference to notes lacking or that the composition is bad in general? If that's the case, I would want you to give me some example of when it's bad (and why if possible) so I can know what i'm doing wrong.
Lastly, I would want to ask if some of this problems are more present in some diffs than in others.
Again, thanks for being honest. This beatmap is something I really want to push trough and that is the only way for me to learn how to do it.
https://osu.ppy.sh/beatmaps/artists/35, go on this link and grab the audio with the correct BPM and OFFSET to make your map, all you have to do is to move all of your diff. I ask you this because it is bad timed. For the notes lacking, I will add my opinion on the diff's section. There is too much things to fix, watch phishifat's tutorial mapping https://www.youtube.com/playlist?list=PLp7-THR1EUHERrIOV4dGClCGGoLIrQtOx.
I'm not gonna make all of your diff yet you have to fix a lot of things, I'm not gonna make things for you. It takes time to make a map and I'm not a modder/BN. If you want to make this ranked, you can let your map in graveyard because your first maps will never be ranked, just map more, look at other people’s maps and try to draw inspiration from them.