but it seems like nothing really gives you enough money besides selling items after battles.
You've answered your own question! You will never run out of battles, so this is your best and most reliable income.
Oh what about peace time? Start a war. In peace time I look at my money and eyeball how much time I have to bleed money away before I must start another war. I do one big circle around looking for wanderers and recruiting troops and what not and end up in the area to start the next war. As long as I have 50K I know I can get the money river flowing fast enough to build it back up before any problems can come up.
So i'm wondering what's the best way to make money besides constantly fighting and selling looted stuff.
There are none. There's only bad ways that waste your campaign time and become incompatible with actual playing the game and painting the map.
Smithing: Make two handed sword and swingable polearms, sell to shop for money, do contracts and so on. PROBLEM: you must wait for stamina to re-build and while you wait enemies re-build parties and move on the map. You can smith n peace time though.
Towns/castles: These should be used ASAP to create new clans so by default having them at all for any amount of time is bad. However if you use a governor with perks they can save you some money IF you have expensive troops AND the governor has matching perks to reduce the cost AND ALSO you can put them in garrison for some amount of time. There have times where stashing expensive mounted troops in a garrison with a governor with "cavalry tactics" perk and some others let me be in +income during a peacetime. But then again, why not just start a war and put that strong force to work for even more money? Get enough towns (and castles) and don't over fill them with expensive garrisons and you will make a good profit..... but I don't want profit I want power! More to come.
Caravans: I want all my companions in my party to promote them to clan leaders ASAP, It's not worth a little extra income to send them out.
Workshops: You can make them, but spending any extra time at all(like coming back to buy thier outputs to sell yourself) on them defeats the purpose of "passive income". You could do literally anything else that need map movement and get more money. But when you have you own faction you might as well make some.
Trading: Even if you know what to trade early game (I don't, anymore) once you have a kingdom you're not going to have time to do trading runs so this fizzles out. Or I should say, you time is 100X more useful to go defeat armies and siege fiefs then to do a trading run. Also this isn't even passive income why did I bring it up? Need coffee.
A GOOD IDEA: Steward perks
and others that help reduce the cost of you daily wages for parties. FOR THE WHOLE FAMILY! Get some of these one or several of you clan mates that lead parties in an army with you and it does make a difference. Sure you will still piddle away money of not fighting, but it takes the bite out of it.