If you already upsized the wire for voltage drop would you have to further derate the wire if you have more than 4 current carrying conductors? Do the calculations compound?

Here's something that might help you:
https://www.electrical-contractor.net/ubb/Forum1/HTML/001967.html

First caculate the load.
Size the wire for the load.
Derate per the table 80%. This will give you a larger conductor. Caculate the VD using this conductor.
If you have to go larger you do not derate again.