hooktheory is also really good. But not free.

In terms of when you spend time on sound design, I've heard a lot of people argue for doing "sound design" first in the process, then moving to arrangement, however to me sound design seems like it should come secondary to the notes..

Also, I've been producing music for 10 years and still struggling to understand, though I've been doing it more seriously now for 2 years...

I guess whatever you're comfortable with. I can't make a melody/chords etc without at least a simple sound that isn't just a plain saw or square; usually pianos, harpsicords or i'll have some patches I've made in the past to use.

That pad in the beginning should have some slow decay. Add some cymbals and hats.
Vocals are kind of quiet, maybe also have some type of percussion, short snares etc in between those chords at 1 minute.
All I can think of for that; this is going really well. Good luck.

To me overproducing is when you don't release a song because you spend all of your time producing it and never deciding where to stop.

Eh, I just think it's where you release a song that has way too much in it at one time.

I started this about 3 weeks ago. I'm a slow worker aside having more duties in the real life.

 You should try adding a short, not too loud 808 snare in place of some of those toms. I think it's ducking too hard during the clap, and maybe take the sub away from the break. I think this track is coming along quite well, I love the chords and also how the toms aren't so loud during the transitions. Keep going at it.

A lot of doing (best way in my opinion,) Seamless, and like above, ADSR.