Why design a tax system at all? It needs extreme detail, but think of it as exploring your world in a different way. Taxes are about history and it’s about goals. There’s lots of reasons for why something may be the way it is in your fantasy land and taxes are a way to show it happening.
Think about something in the real world. Let’s look at Yuan Dynasty, the imperial regime created by Kublai Khan to rule over much of what is now modern China. This was a vast empire and with a myriad of different people. It also had a centralized government which meant provinces had to pay enormous sums of silver to the government in Khanbaliq (Beijing). What to do with literally millions of tonnes of silver? Paper currency. Then, what of their primary source of income? Trade. Thus there were all sorts of merchant fees, tariffs and sales taxes.
Inspectors went out in hordes. China, like other nations back then, had an economy with a backbone in agrarian industries; this meant harvest taxes were still top priority. Gigantic barges full of grain were shipped north and south along the Grand Canal. You can imagine all of these little tidbits come about when a very large and epic setting. Agrarian economy with its harvest taxes. Urban economy with land and property taxes. How people might evade wealth taxes. There’s possible political slants to how taxes work; you can base outcomes on real historical use of a tax system or you can take something to the extreme as sci-fi likes to do.
It’s your world to explore… and tax.