Hello! I've only played the game twice so I'm probably not SUPER authoritative, but
Various shops around the game update as the game goes on, so its worth checking back in to see what new equipment there is. In addition, healthy exploration will give you more armor that you can equip
I'm sure you've heard of the Bravesoul method. By playing Casino minigames, you can purchase an item which drastically scales attack as you lose HP. This is extremely helpful and gets you through some of the more grindy enemies in the game. For characters with AOE attacks, like Jr, it makes many encounters a joke when utilized correctly
Remember that Techs also have a delay built in, so using a tech means your turn will be delayed. This can be fixed with good turn management, as you can always check the HP with the scope equipped, so you never need to put more in than you need.
Finally, look into some ether buffs. Safety level is OP, since it makes sure you will always survive with 1 HP. I use it to fight an optional boss early.
I don't think you can leave the Song once you enter it however, so some of these might not be viable unless you have a backup save.
I remember the Song being really grindy my first playthrough as well. Just keep at it and keep your HP up. There's an overwhelming amount of character customization and systems in XS1, and playing around with the systems and such might help. Experimentation is good.
Hope that helps!