In order to get good you first must get less bad. Since Dominions is a thinking game a lot of this is basically going to be trying to make yourself more familiar with the game.
So here's the two big resources I recommend you peek at.
the first is the manual:
http://www.illwinter.com/dom4/manual_dom4.pdfEven if you read the manual you should have it handy. There's a lot of dense information in there you might otherwise hav missed from playing the game, and sometimes it's easier to understand the manual when you know the game better.
https://larzm42.github.io/dom4inspector/So you read the manual, and wanna try out a nation? This is the mod inspector to help you scout a nation you wanna play as to see what their national units are like without loading the game and setting a game up. Ever annoyed that you don't have easy access to stats of summoned units? Well, the mod inspector also lists those, as well as all the spells. Same for what paths it takes to magic item.
Next up, when playing the game make sure to hit the ? key. Nearly every screen has some handy shortcuts or outright functions you won't otherwise be able to access. Spell lists too big? Hitting r limits things to ritual spells, c to combat spells, and each path can be filtered down as well, with only one filter at a time I think. Tired of adjusting the amount of research points in a school to get the most out your research? The + and - buttons help out in that regard as well, modifying by the smallest possible increment.
When it comes to the philosophy behind god design, you gotta move from thinking that your nation should serve your god. You only have one god, and they can only be in one place and do one thing. A good god serves their nation instead. Does your nation need a spell they can't normally cast? Good scales to bolster their armies? A bless to make sure you have elite troops? You should be asking those questions and more. (sidenote, mostly mentioning this since the game asks you to design a god before you manage to get a closer look at your nation. Feels like it primes players to think about their god first instead of their nation).
Another thing is get used to analyzing and really thinking about the troops you recruit. What do they do, why are you hiring them. Every troop you recruit is taking up valuable space if you have another option better suited to the task. In fact, the AI in dom4 actively ignores some units because it cannot utilize them well. It's not that shocking if you play a game where you don't recruit a commander or unit in that game ever.
Lastly, if you wanna get better you gotta get to the game and play it. And fail at it. Then learn from those failures. When you learn to play well against the AI consider joining a multiplayer game. Those make you really think about what you are doing.