There is a set of rules and it comes from one fact: a sealed slow cooker loses almost no liquid, and it also draws liquid out of everything in it.
On a hob or in an open oven dish, a large amount of water leaves during cooking. That is not a side effect — it is how a sauce becomes concentrated and how flavours become one thing rather than several. In a slow cooker none of it leaves, and the meat and vegetables add more.
So the conversions, in order of how much they matter:
Cut the liquid hard. Not a little — typically to somewhere between a third and a half of what the recipe says, sometimes less. For something like a stew with plenty of vegetables you often need almost no added liquid at all. This is the biggest single change and reducing it a bit is why you saw only a bit of improvement.
Fix the sauce at the end, with the lid off. Take the lid off for the last half hour on high, or lift the solids out and reduce the sauce in a pan for five minutes. This is the step that gets you from cooked ingredients to a dish, and it is the one most people skip.
Season at the end as well as the start. Long cooking flattens aromatics and dulls acidity. A squeeze of something sharp and a fresh grind of pepper at the end restores most of what you think is missing.