All that glitters is not gold. Sure, spreads look tight and many times you get filled in between. But only when it profits Citadel. When the market is about to go the other way they pull their quotes faster then the speed of light (almost). Add that if you post a limit order in the market, you will only get filled when you do not want to, because the dumb (retail) order flow all goes to Citadel.
Exactly. Like I said previously there is a reason Citadel pays for order flow - and it sure isn't because they want to narrow the spreads retailers see - even if some of the time that is the result.