Hey !
You've got some nice heads there, seen them a lot, but in my oppinion the high resolution is holding you back a little.
There's a lot of detail there, that's noone going to see ever, I think the 512x512 limit helps me to find the right path for how much detail is too much and what lines work and which ones don't if that makes sense.
For example, the texture on the ermac leather is probably very washed out in game, while you could introduce a lot of texture if you stayed within the boundrys.
I mean you can even see this effect in your 3d views of the venom head. all those small lines on the forehead are basically invisible ingame