I'll give you some pointers from my own experience and maybe you will find it helpful!
Unfortunately one of the things with youtube is that there is so much bad advice. Now someone might go and say there's not really bad advice that there are just different approaches, some more appropriate than others. But I always try and look for reliable sources from people who have been doing this professionally.
I do some orchestral compositions and I mix and master my own stuff though I come from a rock background as a bass and guitar player. I still struggle to get my orchestral stuff to sound like the pros. I think something is good and then I hear someone else's work and I think "oh my god this is AMAZING" *sigh* But that's always the artist's curse.
I try and spend time listening to real orchestral recordings, my favorite movie scores, and emulating how they feel. Again, since I have a rock background I kept wanting to mix my percussion very up front and "heavy" but it ended up making my mixes sounding thin when they hit a limiter or any sort of bus compression. So given some more studying and messing with EQs you'll eventually figure out how to achieve that sound you're looking for and then you can start to slowly integrate special techniques.
One thing I always wanted to do was wash everything in reverb, but I studied more on this and I think I have improved. So I think maybe that's one thing you might want to look into. There are many ways to achieve "depth" in mixing. Carving out instruments via EQ is one way, but adding the appropriate reverb can be another way. Not to make it "better," but to achieve a new realism and life to your mixes that maybe they are lacking.
Another thing I've always been told by many many composers is to really get the hang of using your MIDI CC messages. Really dig in and use expression to give everything movement. I've had a hard time with this because I haven't spent much time with orchestral instruments so I have to always have a reference on how they should sound (Or at least what I'm trying to achieve). So making sure your automation is killer can be important to the mix rather than worrying about EQ, compression, and etc. And sometimes I forget this and my strings sound flat, or my horns don't have power. Just automation can really make a huge difference.
As far as mastering, a really great plugin is iZotope Ozone. It's got a lot of great tools and can EASILY be misused. But there are tons of tutorials on how to use it's advanced features to make professional sounding masters (That is if you don't already use it)
And of course, bounce your mixes off of someone else! I have a few friends I always go to, most of them are mix engineers, live sound engineers and don't really do composition, but they have an ear for a good track so it's good to get another perspective. Sending them to random people you don't really trust can be hard and subjective and it may be hard to tell where you need to go with your mix.
Anywho, I hope this has been helpful!
