2. Try highlighting and washing your models, it creates depth and realness to them. As of now, they look flat; gloss won't help much if all it does it makes your model more shiny.

3. The tyranids... I'm not sure how to say this but I think they are not properly defined, it looks like you haven't finish painting it (or if you have not, then please ignore the following).
For example your Tyrannofex, if I'm right, your paints were too thin and had added a lot of water, this can be seen as the carapace is not evenly painted, there's patches of paint on it that I believe it's because paint accumulated there? Try not to soak too many paints onto your brush when you paint and a mixture of 1:1 between water and paint should be sufficient.
The other thing would be the gun (the head?), the scything talons and the tail are of same colour. In my opinion details like claws, talons, flesh/muscle and bones should be of different colour or properly defined; the only separation of parts/bitz I see in your model is the carapace and everything else. What I would suggest is to look at GW painted ones for reference, or if you would like to, my painted Tyranids although it's no where near masterclass standard but I try to differentiate/contrast each unique parts of the model.

The overall quality of your models can be enhanced as long as you remember the basic techniques.
1. Paint layers of evenly mixed paint with water. Layer"s", not one thick layer.
2. Wash your models (holes, gaps, panels, crevices) with either GW washes or DIY washes. If you're using GW washes, water them down a tiny bit first.
3. Highlight edges of your model, parts where it would appear brighter and parts where it would glow, etc. your vehicle engines and weapons such as the fusion cascades on your XV9.
Please try looking at the tutorials at ATT itself, I've learned much from there.

That is a very cool AA gun. Far more Tau-ish than a quad auto cannon.

Id consider shamelessly stealing the idea but looks like the project chewed through quit a few burst cannons which I just don't have

This burst thing is meant to count as a quad gun; I do have an actual lascannon lurking somewhere at the bottom of my bits, so any ideas on how to Tau-ify that would be appreciated.

Any ideas on Tau Aegis lines, as I asked? I've seen suggestions of barricades made from drone saucers from the Apocalypse rulebook IIRC, but I don't have enough spare...

Do you have the Cities of Death book? Maybe a line similar to the one on page 69? It's a little big and uses a lot of vehicle pieces, but it looks really nice.

Polystyrene coving? You can get big stuff that'd be battlesuit sized (10cm) and miniature coving that's only 1-2cms high.
It'd be around £3-5 per meter, and fairly easy to cut and score panel lines into. Also comes with ready made inside and outside corners.