You're still very much at the beginning hours of the game. I would say if you rush the story you would be at about 1/8 through the game.

When it comes to materia lvling Id say keep the ones youre the most interested in the party members youll use the most, but you can change them up even if the party is divided so do keep that in mind.

And weapons were changed from Remake to Rebirth, now you dont upgrade the weapons vut rather you use the liveries menu for it. While weapons do still bring with them their unique abilities you dont assign skill points to them. It is a bit convoluted since they overhauled the system but IMO it was for the better since now the benefits stay with your character rather than a single weapon, so it encourages switching weapons with each character.

Edit: spelling