Chemically speaking, why do you have to add chilli pwder earlier rather than at the end?
Like you would describe colour or music, you add dark at the beginning &amp; bright at the end.

Any type of deep curry or chilli flavour will improve over several hours in the pot, &amp; even overnight, left to go cold then re-heated the next day.

The problem with that can be that you lose some of the 'brightness' off the top; some of the zing is missing.

So you add additional aromatics at the end.
Whether that's some garam masala in a curry, or some extra chilli powder [I favour New Mexico Red for warmth, or some chipotle or smoked paprika for aroma, or just a bit of generic cayenne if it's not quite punchy enough] to a chilli, the purpose is the same - to put back some 'highs' in the flavour palette.
If you need timings, add anything like this when you put your rice on - so you're about half an hour from serving.

Right at the very end you then add your brightest, most delicate flavours - a handful of coriander [US cilantro] or some flat leaf parsley, topped off with a squeeze of lime or lemon.

&amp; there you have a symphony - all the colours of the rainbow.

If you're getting "bitter" or "grainy" you're adding the wrong thing, or trying to add "all flavours" right at the end, which isn't going to work.