I pretty much agree with the majority of the responses here. Explain Haki in its entirety from the start. Remain consistent with that structure and never deviate from it. This is what Togashi did with Nen and it's no wonder that Nen is widely regarded as the best power structure in all of Shonen. Info dumping right away reduces the ambiguity of the power and makes things a lot more clear for us readers to follow. It also reduces the amount of bullshit we keep getting with Haki every new arc. You need to be calm to use CoO, you can now sense emotions and feelings, haki now blooms in stressful situations to make it super convenient for the protagonist to get a power-up.
If you're going to introduce advanced versions of haki then you have to follow the same structure. Info dump everything from the start. Togashi introduced Ko and Ken after already establishing Nen and it's worked out completely fine. Furthermore, make it more limited. As in, reduce the number of ppl who have. You might say that it's already limited with only top tiers and a few high tiers having it. But relative to the story it isn't. We're at the stage now where we're only seeing these top tiers and high tiers so it appears abundant. So, how do you limit it? Give it to non-df users. Shanks, Mihawk, Garp, and Roger. Maybe just a select few–like one or two–df users can have it. Prime Whitebeard and Sakazuki for example. High tiers most definitely should not have it. By limiting the advanced forms of haki, you make it more unique and special. It hypes up the characters that lack the DFs a lot and it hypes those that do have a df and the advanced versions as well. Moreover, it makes Luffy obtaining it to be fruitful and a bigger accomplishment.