Lots of good advice here for sure. I've tried most of the things you've suggested. Rendering animations without background has been extremely helpful. Doing that for a whole scene (such as that big farm scene with the horse and the girls) gets quite high in workload though. Some 290 images that need to be edited... Plus, I find lighting to get really wacky sometimes, since the shadows don't really make sense and the light changes a lot when there aren't items to reflect light back, etc. This can be noticed in the animations, where you suddenly see the lighting change between the picture and the animation itself.
I'd love to know more about post-processing. I'm using Gimp but got 0 experience with color grading, vignetting, and such... My approach to the game has been quantity over quality in many ways... When playing other games here on F95, I always feel like there is way too much text and way too few images and angles. So when creating my own first game, I tried an approach that at least I would enjoy, if that makes sense. I do appreciate the constructive criticism though, and I'll try to keep it in mind going forward.
Well for an easier Workflow over multiple scenes it comes in handy to limit yourself to just a few lighting scenarios and settings, like intensity direction and so on. Then you can recycle more even in still scenes. Furthermore I would drop that horrendous camera ring light a lot of DAZ tutorials are propagating (if you use that) - it looks awful since it removes a lot of model details and messes up your shadows, being an additional light source, proper scene lighting works better.
To help out with shadows, when rendering multiple passes you could look up some shadow catcher tutorials on youtube - the software used doesn't really matter the principle is the same. The idea is you have a less detailed models, which are not visible in the render only doing a shadow pass you can then integrate in between the background and the models...
For budget composing and post processing, as in free, I can recommend Krita on the still image site - you can follow pretty much any Photoshop tutorial, if there are no Krita specific ones for the given task, YouTube is your friend here as well. Checking out some photography and videography tutorials is helpful as well to learn some tricks on making stuff look great.
For set up once, perform on many images processing XNView is great, I use that regularly when I need to adjust project assets. (One got to deal with horrendous stuff, when dealing with app and game development, getting assets not in spec regarding size, file format, color settings, etc.. And it's faster to fix it yourself then telling the desinger that the 200 images, which just came, are all set up wrong.)
If you're not afraid of learning some stranger-to-use stuff I can recommend Blender - you can compose and rework/grade a shit ton of images in one go from multiple layers.
You just have to prepare some "workflow" steps, e.g. take layer-image-a multiply layer-image-b and c on top of that, adjust color, add vignette and save as image sequence to that folder. Works great when you sequentially name your renders and store them into one folder, e.g background_001, shadow_001, characters_001,... it will automatically notice it's a sequence and applys the whole prepared thing to all the sets whith the matching numbers. In fact it handles the image collection as "video" but can save the result as image sequence instead of a video file, for actual animations you can choose a video file of course. Just look up Blender compositor.
If Blender isn't too hard to learn for you (it's definitely not easy, but not that hard and you manged to get a hang on DAZ already), I would even suggest using that for rendering.
Exporting single models up to whole scenes from DAZ to Blender is possible, officially supported and well explained in several tutorials (YouTube again for example and the DAZ documentation).
I actually prefer setting up the models in DAZ (when I wanna use some that is) and doing all the rest, including rendering in Blender. The Cycles render engine produces much better results, especially with hdri maps for realistic lighting. Using the Eevee render engine produces really really quick results, depending on your hardware (yes a proper GPU is a good idea) and scene complexity nearly in relatime, but they might not look as realistic as in cycles. Compared to DAZs render engine Eevee might hold up well and DAZ models are in the semi realistic, stylized realm anyway, so might be an option.
Doing the stuff in Blender enables rendering, denoising and post processing in one go, while it's easy to render out different layer passes as well and have them separately stored for later adjustments. As a Bonus there are loads and loads of tutorials on how to do stuff with it, on multilayer workflows as well.