How to make the 3rd raid difficulty work

[Cetonis]

players don't need any carrot-on-the-stick reward scheme

Yes they do. If they didn't, more people than virtually-nobody would be going back and trying to min-sync clear A4S and A8S, or do the former with Nisi since they probably didn't originally. Or heck, min syncing SCoB Savage. Challenge in and of itself doesn't drive people enough to put in dozens of hours of time on a fight.

Having a gear reward still matters, at the end of the day. An i280-equivalent weapon would be "vanity" for most intents and purposes, but would feel far bigger than that and would eventually have a real value of some time savings down the road. It wouldn't be any worse than i275 weapons are now, and nobody complains about those.

Also, the use of such a reward can increase the content's staying power, especially if it's locked to the class you cleared on as described before. Theoretically that could be done with level 1 weapons too, but you'd be relying a lot on people universally getting super hype over the looks of every piece, which is obviously impossible.