Ok, thanks for the answers.
I just want to make sure, before submitting feedback, that I didn't merely fail to find how to do a thing. Though obviously, if I fail to find how to do something, it somehow means that instead of a lack of feature, it's a lack of the game teaching me how to play it.
Also, the hotbar is a big bad design at the moment, and I hope Larian changes that UI system in depth. So much so that switching icons from one page of something to another won't be remotely needed.
But still, I wanted to check before complaining about how impractical the current system is.
(@CMF : yeah, I deactivated all auto-populate. Still, for a character like Shadowheart, between her spells and some food, I tend to haver her hotbar full.)