What went wrong is that you mixed up your terms and made the wrong recipe.
There are many custards. The common thing is that they are all a liquid thickened by mixing it with egg yolks and cooking them until they are just right. But from there on, you get countless variations, and they can have different names.
The name "crème anglaise" is used for a liquid custard. It is somewhat thickened (not as liquid as pure cream), and is used as a kind of sweet sauce, poured over pastries. A typical use would be to eat it with an apple strudel.
Yours turned out more liquid than in the original recipe because you had 40 g less fat and 40 g too much liquid. But even if you had not done that, the recipe is still expected to stay runny.
If you want to fill a tart, I suggest that you use a different custard, for example a crème pâtissière. It is a cooked custard (not baked like a flan) and sets as a spreadable/pipeable mass. It will use the same ingredients as a crème anglaise, but in different proportions, so the taste will be very similar.
The "don't do substitutes" thing still applies. In baking, the amount of fat and sugar in recipes is what creates the final texture, and cannot be changed due to dietary considerations. If you try, you will end up with food which is too dry, or too runny, or in some other way different from the original.
