After thinking about it, there are some bigger issues that come up when you axe the opening space battle. As much as I would like to be able to cut even more of the runtime down, I don’t think that is a feasible option for multiple reasons.

First, you lose the logical and emotional introduction to Rose as a character. I don’t see how you can introduce her crying over her medallion if we have no idea why, and then we are somewhat confused as to what her past is and what the Resistance means to her. We don’t have sufficient motivation for any of her actions.

Second, you now have more issues when introducing the character of DJ. Under the idea of removing Canto Bight entirely, we first see him on the way back to the Resistance. His introduction would then be him asking for a price and then Rose giving the medallion. Sadly, you’d have to lose the stuff afterwards of him stealing the ship and talking to Finn since they refer to it as a stolen Canto Bight ship, whereas with the changes, we would assume it as a Resistance ship.

But if you cut out the foundation for the importance of the Medallion, then none of it really makes that much sense when you see Rose hand over the Medallion and then her getting it back. As an audience member, you have no idea what it means to her as a character.

And Lastly, you lose the initial setup for Poe’s character arc. This one doesn’t mean nearly as much to me, cause you can still have his arc begin when he defies the power of Holdo. So this is more of a nitpick, but it would still be worth losing it ultimately to give more focus to the A-plot of Luke, Rey, and Kylo.

But the the first two issues seem like pretty big obstacles to actually removing the opening space battle and starting on Ach-To.