Character

By late game you'll necessarily be using several cross-hotbars, there's just no way around it. My scholar UI uses seven because of some specificities of the class, I'd say 3-4 is typical.

TIP: You can switch between hotbars in two ways:
- tapping R1 cycles through all of them,
- holding R1 and pressing one of the main 8 buttons will get you to a particular bar (try holding down R1 and the game will display a button mapping).
I only use the second option, because the first gets very clunky with more than three bars.

TIP: The right side of hotbars (cross/square/triangle/circle) is easier to use while running, it's where you should put things like instant spells (mages), combo starters (melee), and generally the skills you use most. You'll steel need to claw from time to time, though.

SETTING: Getting more space for class skills
By default, crossbars 4 to 8 (or something like that, I don't remember) are shared between all your classes. That means you only have 3 bars to use for your class skills. You can increase that number by unticking bars in the "Character Configuration -> Hotbar Settings -> Sharing" tab.

SETTING: Disable the pet hotbar
Pet using classes get a special hotbar for pet-related actions. This bar has a huge tendency to screw you up, because it messes with your ability to switch freely between the others. The first thing arcanists will want to do once they unlock their pet is disable the pet bar and place the relevant actions in their standard hotbars instead.
The setting to disable the pet bar is in "Character Configuration -> Hotbar Settings -> Cross -> General Settings". All pet-related actions can be found in the Pets section of your "Actions and Traits" menu.

SETTING: Enable expanded hold controls
I couldn't find a short descriptive name for this setting, and apparently neither could the devs. It allows you to press both L2 and R2 in order to access a specific hotbar, no matter which one is currently active. I can't overstate the usefulness of this option - it doubles the number of skills you can access without a hotbar switch.
The option to enable this is in "Character Configuration -> Hotbar Settings -> Cross -> Expanded Hold Controls". You'll also need to choose which hotbar L2->R2 and R2->L2 get redirected to; you can set both to the same bar if you think you might get clumsy.