But Luke never actually TELLS anyone about what he plans to do on Crait. Not even Leia, even though he stops to "chat." They're all just standing around watching Luke do his thing out on the salt field - totally unaware he's about to go *poof* and disappear, meanwhile... they're all still trapped in the base and not escaping presumably because they want to see this epic "Luke saves the day" showdown. Poe just so happens to figure it out in time (in a somewhat Scooby Doo Fred/Velma-revelation moment.)
The issue isn't even just the holo-projection thing, which has a host of issues centered on perceived cowardice, logic errors and violation of established saga rules (as seen in many debates on here) - it's the whole context in which it's used also.
Yeah, I concede the point that having Poe figure out Luke's plan in that way was ill-conceived. The only justifications I can offer would be mere conjecture. Maybe Rian Johnson wrote the scene thinking that Luke would assume that the Resistance folks wouldn't be so stupid as to just stand there instead of taking advantage of the pause in the First Order's advance. I mean, seriously, if they just saw Luke walk in from somewhere in the back, and still decided to just stand there as spectators instead of trying to find a similar way out, then they deserved to be beaten.
It would have made more sense for Leia to tell them what Luke was doing. When I watch Luke hand Leia Han's dice (and I'm starting to hate those dice), and she has that astonished/surprised reaction, I thought it was because she realized that he wasn't really there since she couldn't feel his hand. At that point, she would've been able to put the pieces together and get them to focus on escaping. But, again, the writing here left something lacking because we're not even sure if Leia knew that wasn't really Luke. And she wasn't being proactive; as you noted, it was Poe.
The writing was clunky in several parts, and I'm not going to deny that. I think Johnson wanted to make a Star Wars movie that could also stand on it's own as a great and well-written movie. Frankly, I don't think he's good enough (or proven enough) to have that type of aspiration. But, for me, the Force projection was a highlight. I appreciate that device because it worked to keep the audience guessing, it showed Luke mastering the Force, and it let him best Kylo on a couple of different levels. I didn't see it as cowardice to use the projection; I saw it as Luke's best chance at redemption without giving Kylo what he wanted.
In coming to terms with Luke not actually going in person, I asked myself: what would have been an alternative that would have better served the story? Would it be Luke taking down the whole First Order? That'd be a bit much. Luke defeating Kylo and slaying his nephew? I don't think so. Luke turning Kylo back to the good side? Derivitive and lame. Luke getting killed by Kylo? No thanks.
I know that most fans (yourself included) don't see Luke's storyline in TLJ the same way I do. I respect that. And I recognize tons of problems with TLJ too (injecting SJW nonsense was awful; abandoning Force Awakens storylines annoyed me; Leia Poppins was so regrettable; Canto Bight was as bad as anything in the horrible prequels). But, I overlook a lot of flaws because I appreciate such a stark departure from the pandering and often embarrassing PT. I wanted this material taken seriously again. Even when the writing in TLJ failed, I never questioned the intent to deliver something that sought to elevate Star Wars. And, for me personally, the Luke Force projection was an example of when I fully appreciated the end result (even if still flawed).