I honestly didn't mind him not being there, as I think it would have confused people even more as to why they are all camping and doing leisure activities during the game sometimes, especially after being there firsthand.
Yeah, considering the tone they were going for with the road trip, I think the game might have actually been better served with an even lower key opening.
Imagine if you arrive to Galdin Quay to find Iris frustrated that the road to Lestallum blocked, and your first major task is to take down the Niff blockade to escort her there. There's a bit of mystery in that the boys think it's weird that she left the city even though she wasn't sent to the wedding, but everything's still light-hearted since the city hasn't fallen yet.
Once you get her there, you get a cutscene showing that Luna is actually in Insomnia. Then Cor could ask for help investigating another new Niff base (the one with Loqi) in Leide and hint at tensions in the city. He takes off once the mission is complete, and the boys decide that they need to check up on the city.
The imperial blockade is already in place so they fight their way around, but night falls and they have to rest at the haven overlooking the city. And then they're woken up in the middle of the night by the sound of explosions and gunfire, and as soon as they try to get into the city, Cor shows up to keep Noct from getting himself killed.
Noct is sent to collect the first two Royal Arms in Leide, then he returns to Lestallum to check on Iris, but has to collect the Royal Arm in the cave and gets sidetracked by Ardyn before seeing her.
Everything else can proceed from there. Noct fights Titan, loses the Regalia, gains Ramuh's favor, and gets the Regalia back. Jared's death is replaced with Iris' reaction to the destruction of Insomnia, which should hit harder and make the revenge quest make more sense.
Everything after that can basically remain the same, and very few assets would even need to change overall. Maybe some of the post-Ch. 9 banter changes were implemented early to match the change in tone, but that wouldn't require new assets.
...It'd be nice to get further explication and maybe a few more areas to explore in the second half (namely an open-ish area around Cartanica, a Tenebrae dungeon, and a few cutscenes explaining Ravus's deal) but that just requires more stuff instead of structural changes.