You can definitely get most items from normal gameplay. High level raids are entirely optional.

On that note, you'll see that some types of content in the game are labelled "raids" but (with the exception of the Binding Coil and savage difficulty raids) are no harder than regular dungeons or trials, so don't get scared off by the term. Alliance raids are more about scale than difficulty, and eight-man raids have a basic "story level" difficulty before unlocking the more challenging savage mode later.

MSQ is the core of the game that unlocks access to new areas and most of the dungeons. You should also keep up with your job quests, as they unlock skills, and sidequests with a blue "plus" icon, which indicates that it unlocks something – some useful game features are locked by these quests, like the challenge log and battle chocobo, but more often they unlock optional dungeons or (in the post-credits phase of each expansion) entire side stories spanning multiple dungeons or trials.

While optional, some of these side stories tie back into the main story, besides giving you access to new dungeons.

Glamour is everywhere and any piece of equipment can be used as a glamour. Some are locked behind specific types of content but there's a huge variety available from just normal gameplay. You can buy gear, receive it from quests or dungeons (every dungeon has unique gear) or various side content like PVP or the Gold Saucer minigame hall.

The only glamours locked behind high level raiding are a few specific weapons and dyeable versions of outfits you can obtain (in undyeable form) from the easier version of the same fight. And once they become outdated content, you're able to go in with an unsynched party – keeping your current level instead of being synched down to the original level – so you can get the things eventually even if you don't want to learn the fight when it's current.