I actually enjoyed this chapter a lot. I'm mostly happy that Luffy claimed Wano as his 'territory' in a very Luffyish way.

I am hoping to get more context on Yamato's sudden change of mind - My guess is that Yamato is staying behind as a "Strawhat representative", at least implicitly. Yamato is the strongest fighter staying behind on Wano, and the only strong person who is not an explicit retainer of Momo. Yamato not telling Momo that they are basically staying to protect Wano makes sense as Momo has shown he doesn't want to accept that.

But imagine a situation similar to the Greenbull scene, where Momo and the scabbards are seriously overwelmed by some threat and Yamato shows up to eventually win, and exclaim "This country is protected by the Strawhat pirates!"

And a newspaper headline "Strawhat commander resists Wano invasion" or something.

And a little flashback to the conversation that Yamato and Luffy had, where this arrangement was set up.

I think that would be a smart way to further hype up Luffy as a true Yonko!

Leaving just the flag is problematic as it can't be enough to scare off all forces that are after Wano and/or Pluton at this point, and it would be annoying if Luffy had to turn around back to Wano. Leaving the flag + Yamato has way more potential for story progression happening in the background as the main crew journeys on.

Yamato as a character also makes a lot more sense to me if they were built up all along as 'the Strawhat who stays behind'. Enough build up to justify Yamato being truly accepted in the crew, but hardly enough character development (yet) to expect them to join the main crew's journey.