How to increase Thai curry flavour without increase spicy
I did a search for Gelbe Curry Paste and found the product in your pic in a result from Amazon.

Based on what I read in a review, you add coconut milk to this curry paste. Therefore I'm hopefully correct that you are toning down the heat and flavors by adding more coconut milk or other milk. So after you tone it down, you want to add flavor back but not add more heat.

The product description lists the ingredients as:


  Product of Thailand Dried Red Chili, Shallot, Garlic, Lemongrass, Salt, Galangal, Tumeric, Cumin, Coriander Seeds, Spices


You can add flavor back by adding some of the listed ingredients (to your liking), but not adding more chili. In the US ingredients are listed in order of amount, highest to lowest. While this may or may not be the case here, it may help with deciding which to add more of.

You'll likely have to play around a bit, but hopefully you can get that flavor back.