Welcome to games of this vintage
There just wasn't enough room on the cartridge to do a plethora of combat themes, that was pretty true of every Squaresoft RPG at the time. You had the main battle theme, the standard boss theme, maybe a second or third boss theme for significant/challenging moments, and then the final boss music. It was incredibly rare for anything more than that, and if it did then you usually had something special.
The music may have been weaker on the whole since it was Shimomura's first solo outing and I think this is where her more playful style originated which would continue onto things like Legend of Mana, Kingdom Hearts, and Mario & Luigi games.
Lack of inventory stacking was indeed odd since Squaresoft had done away with that in Final Fantasy by the fourth (which stacked items but you still had a hard limit on stacks) or fifth game. At the time it didn't phase me in Mario RPG because Earthbound had come out the year prior and that was even more restrictive since it wasn't a shared item pool and you had to shuffle items between party members with a fairly tight limit of what you could carry.
What makes Mario RPG particularly curious is usually when there's a limit on what you can carry you still have a "warehouse" mechanic like your sister in Earthbound or the PC in Pokemon, but Mario RPG lacks that. Which makes me wonder if they had plans and dropped it at some point or just ran out of time to implement something like that. At the same time, maybe it's a balance thing as the game tends to give you back a copy of an item if you use it so they probably didn't want people walking into combat with stacks and stacks of items and just spamming them to clear some of the harder encounters.
Anyways, sounds like you had a good time regardless!
Culex and ???