Some general ways to cut polygons:
1. Cull your curves.
Look at every curve on your model and ask your self if it is really needed. A lot of curves are just too small and tiny to be noticeable on the game scale. Like the edge curves on your door window there or the gun barrel. You can make those square without anyone noticing the difference.
2. Simplify your curves.
Some curves, like the edges of your doors there you'll want to keep. But you can simplify them a lot. A curved door edge looks just as well in 8-15 sidesas it does in 24 when you are at civ scale. For example on my PT-76 model the turret (the most visible round thing) only has 18 sides. The barrel 12 and so on. And those are huge visible parts. The front and rear track (the place they bend) only have 5.
View attachment 507501
Exbit 1: Ugly zoomed in, looks great in the game.
3. Erase invisible faces.
There are plenty of faces on your model that are newer going to be visible in the game. Examples of these include but are not limited to the bottom of the hull and tracks, the undersides of turrets and other components which are stuck on top of your model and literally every face that is stuck inside or up against another. Like the rear of the gun barrel for example. And even though you can't see them these faces still exist on your model, still take up memory and if the game is badly optimize might even still be rendered! (and even if not, culling them in the game takes time) only to be overwritten later screwing performance in the process. So you should seek to find and nuke them.
Basically every time you know a face is either on the bottom of the vehicle or pressing against/inside of another you should delete said face. Also, pay extra attention when merging multiple objects together as the edge faces won't vanish without you deleting it. Finally, learn to use and love the Weld vertices function. It merges vertices (the tiny points that form your corners) together reducing the vertex count significantly when used right.
This is usually the last step I do before exporting
4. Use textures liberally.
Not every surface feature on your model needs to be a real physical thing. At the scale models are displayed in civ you only really need the big chunky bits to look right. And even with those you can often get away with moving them into a texture.
Like the tracks you are using. Imagine if those were actual wheels modeled in 3D, let alone if the track links were individual objects. And the logic goes on. A door or window, the opening in a muzzle break, the details on your guns, flush hatches and surface panels and all sorts of other things can be made better by simply not existing physically.
What more, because you can use 128x128 or even 256x256 textures you can get more quality out of them than you could a more space consuming physical model.
Bottom line is, seek to move as much of your geometry into the textures as you can and only model the big chunky bits. It saves size, complexity and labor.
5. Optimize using Nifscope.
Nifscope has two great optimization options hidden away under the spells menu. These are Combine Properties and Remove Bogus Nodes. They help get rid of unnecessary nodes in your NIF file thus reducing size.
The last thing you should do with your model before publishing it should be to hit these two options in that order.
PS:
Feel free to look at my PT-76 model for inspiration. Also, I suggest looking at the vanilla Galleon and Frigate models for some inspired optimizing on the devs part. Also the Trireme.