I only ever found the magic classes complicated when I was new. The limited spells per day and number of cast rules, what you had access to as what, memorizing, etc... All a lot of work compared to other games I was used to just having a mana pool.
The fact that they let prepared casters change prepared spells without a long rest is a HUGE buff. As written you pick your spells when you long rest, and if it turns out you needed something you didn't prepare you'd be out of luck
The fact that you can long rest basically whenever is a huge break from D&D's expectations. Namely, the adventuring day. Granted, D&D's adventuring day is hot garbage so I see why they didn't stick to it.
But you're right it doesn't make a huge difference in BG3, outside of the few timed events, but it is a buff.
There are certain timed events where a long rest can change the outcome, so in those cases it's very helpful to be able to prepare if you encounter a situation that you're not ready for.
5e (and BG3 in turn) even simplified Vancian casting compared to previous versions. Used to be that you had slots that you prepared a specific spell for. If you had four slots and needed to cast Magic Missile three times, but you used two of those slots to prepare Grease and Fog Cloud, you were out of luck.
It... Does. I enjoy having spell levels and slots way more than something like mana. In any system with mana I almost always use either the most dmg/mana efficient spell or the big one, etc. Having spells be divided in levels and action/bonus action makes for a much more interesting combat.
For example, I much preferred the way DS1&2 does spellcasting compared to DS3 and elden ring. You have a limited amount of casts and the system forces you to either have a varied spellcasting repertoire or to find your way to get several of the same spell to ge more uses. Mana is... mana. Boring.
Tangent to that is ability cool downs like Divinity original sin 2. It's the same as you said with mana, if you have to wait certain amount of turns to reuse your abilities combat revolves around min maxing your ability cool downs. Using long cool downs first in order to be able to use them again on the same fight. Buffs need to be recast every fight instead of after a long rest. Combat ends up being the same for every fight since you're using the same abilities in the same order to maximize the cool downs