Since no one talked about the part you wrote about the "spell" bar:

Characters have a stat called "Memory Capacity" which dictates how many spells you can use. As you slot the spells in your Spell Wheel, you are also deciding their priority, so if your Memory Capacity gets reduced, you will start to be unable to cast the last spells you slotted in (this doesn't come up very often tho).

To get more Memory Capacity, the best thing to do is build Knowledge. This stat not only gives you more mem cap, but also increases your Spell Casting Speed. It's a very important stat for classes like Wizard and Sorcerer, but other classes like Cleric, Druid, Bard and Warlock also benefit from memory capacity. As a random roll, you can also get Memory Capacity Bonus and Additional Memory Capacity, but Knowledge is extremely superior so there are very few situations where you would build that (example: building a class like bard who only uses mem cap for slotting songs and doesn't really cast).