Well an airbrush is good on larger models, of models of such a small scale they are much harder to use effectively i think (admittedly i've never used one yet).
From what i've seen of your models you have a good eye for colour and composition but it is the transitions which are letting you down a little. The most evident is on your Imperial guard sniper, his green cloak for example you can see the changes between colours. I have found that the key to getting really smooth transitions is patience and to use that time to apply the techniques. This little gem of a tutorial by Mahon is possibly one of the most useful pieces of information for those who want to improve, i know i've learnt alot from this;
https://www.chestofcolors.com/forum/view ... =21&t=5300
One other thing i've noticed on the sniper's gun is that the surface is a bit rough, that can be down to two things, first is that the surface was a little bit rough, which is down to pre-paint preparation of the mini. The other is that the paint maybe a little too thick, so try thinning your paints a little now and then and create colour using layers rather than one big go. I have the feeling it is the latter because alot of my minis were like that for a long time too.
I did notice that many of your models have a glossy sheen to them, can i recommend you either add a matt varnish to some of your layers of paint as you're working on them or is that an effect from the finishing varnish you use? Maybe it's even a camera artefact? (im horrible for those).
As for primer, i think it is mostly down to personal preference. From what i've seen the majority of painters use black primer for gaming models and white/grey for display stuff. However, it is personal choice, like i prefer using black primer in general, even for display pieces.
I hope this is all helpful to you

oh and just keep painting and showing it to people to get comments on how to improve. Like Mahon said, you're at a good stage where you know a good mini.
