The first big problem I have with the UI is that there is no way to check what a model of car actually has access to so that you know it will be built with the features you want to include.
Right now you may have done the research and set the model to be built but if you missed 1 upgrade that you are able to purchase it doesn’t inform you of this. It would be nice if the game highlighted the stations in question and gave you an easy way to upgrade all the things you need to make that model happen.
The thing that I found really annoying was trying to figure out what stations I needed to upgrade and when I did upgrade them, the UI would show a pop up and remove the window so that if I had to purchase multiple upgrades it would require me to find that station again and upgrade things 1 at a time.
The second big problem is the way things are organized into groups. For instance with the slots, I would think that tires would be under fit accessories and not fit engine. The main issue here is the research window requiring you to go from one tab to another to progress down a single line of tech. I think this is not a good way of doing it and should be grouped so that its more linear but separated by category instead. Again wheels being under engine specialization research is kind of confusing.
I would suggest going about it like Hearts of Iron 4 and have a single linear tech tree for each category of parts, the first tab being the one that allows you to unlock the other tabs. I feel like this is more intuitive way of doing it and is a lot less confusing and going back and forth to complete a set of research. So each category like accessories, body, engine, chassis, paint would all be under their own tab and at the bottom of the tech tree in that tab would be a way to unlock technology relevant to that category. This means that, while there may be a lot of tabs, you should be able to fit everything onto 1 screen so that everything that is possible is visible immediately. A lot of time is spent on the research screen and currently its my least favorite part of the game because you have to hunt for things to get anything done, and I find myself dreading having to restart a new game to go through it again.
Maybe include a factory management tab somewhere where you can manage things like upgrades, research, design, power and fire the employees or hire them back as needed if you are low on money.