How to add creaminess to cooked corn chowder; mine is too thick?

I don't know if you'll be able to salvage this soup, but I will tell you how you can try. The reason your soup tastes unpleasant is that you added a raw flour slurry. Adding raw flour gives a raw flour flavor which is unpleasant as you've found. In small quantities you may not notice it, but if you add a lot it can ruin a dish. If it is very strong now it may still be too strong after you thin the soup out.

The only way to thin the soup is to dilute the soup with fresh milk and/or stock. What I would try is to remove half the liquid (strain out the solids and put them back in) and replace it with fresh liquid and add some herbs and spices to replace those flavors. The reasons for removing half the liquid as opposed to adding more to the whole soup are 1) to keep it in balance with the solid ingredients and 2) to minimize waste: if it doesn't work you're either throwing out double or if you decide to eat it you have twice the unpalatable soup to eat. Bring it up to temperature and try it, it may need more seasoning. Try some white pepper, it's good in chowder and may help to disguise any remaining raw flour flavor. That's as far as I would personally go, if you end up with something edible I'd stop before you go overboard and make it worse.

There are 2 main ways of thickening a soup, one is to use a slurry as you did. Slurries are quick and convenient, if you want to use a slurry use cornstarch (corn flour) or potato starch instead, they will thicken without adding flavor. The second method is to make a roux, which is equal amounts by weight of flour and butter cooked in a pan. The benefit of a roux is it cooks the rawness out of the flour and adds flavor. The longer you cook it the darker it gets, the more flavor it adds but it reduces its thickening power. For my milk based chowders I make a light roux, cooking it just a minute or two as I like the depth of flavor it gives. However, there's nothing wrong with a cornstarch slurry if that's what you have the time and inclination to do, a roux is just extra credit. If time is a factor you can make a roux ahead of time and keep it in the refrigerator.

Another problem you ran into was that in a slow cooker it can take a long time to see the results after adding thickener, this is because slow cooker temperatures are low and flour/cornstarch don't fully thicken until the temperature reaches 203°F/95°C, although they start to thicken at a lower temperature. You have to know how much thickener to add, or you add it and then wait a long time to see the result. You could also use a lower temperature thickening agent like tapioca starch or arrowroot and see the result sooner.