For $500 I'd look at a VFC, the G&G top tech you mentioned is a good gun internally and externally, comes with a lot of internal upgrades above the KA M4 people seem to be telling you to buy. I've owned a KA and externally they are great, internally they are okay, with upgrades they can be great guns, but I personally prefer to spend a bit more up front for something that will be more reliable out of the box. Don't bother with a CM16 if you aren't on a really low budget, it is a decent starter gun but it will need upgrades and the body is plastic.
If you like upgrading your own guns go for the KA M4
If you want some upgrades done for you out of the box go for the G&G
If you want a mostly upgraded gun for not much more than the G&G look at VFC
Also welcome back