On that "DS looks way too small" issue, I think what everyone's said for a long while, about having the reveal of the new DS dwarfing the Executor, is ideal.
Also, maybe try to minimize the number of shots in the film that actually feature the DS and the Moon both, since those are the ones that make the DS looks small. Instead keep the DS as the focus, dwarfing everything else, instead.
From that scale picture, it seems like the Executor crashing will no almost nothing to the DS, so I don't know, maybe you'll have to make the ship going down more meaningful than it actually crashing. Like those suggestions about Piett deliberately crashing the ship in a kamikaze attack (however futile).
In general, actually, you don't get a sense of hopelessness from the DS in this one. It's even bigger, it's shielded, and it hasn't got an easy thermal exhaust port to destroy it. It ought to be invincible and unstoppable, like nothing they can do even scratches it, not even a SUper Star Destroyer crashing into it. As it is right now, as soon as the shield is down, it's easily blown up by a couple of tiny ships. So much for unstoppable. It robs the film of its potential impact. You should be on the edge of your seat, wondering how they're going to survive against this invincible DS, not waiting for them to destroy it.
I like the idea of the Executor doing only small damage, but exposing a way into the core for the Falcon, only exposing a weakness at the last minute, not part of the plan from the start. Instead maybe the plan could have been to literally just blast the crap out of the station while its unfinished, not operational and undefended, to make it unusable before its operational (with the consequence of it becoming operational meaning the end of the Rebellion, no question). Hence the entire fleet coming. It raises the stakes and feels like the big finale.