Most of these work with keyboard but not with only mouse or only touch on mobile, need additional UI hints to be visible to the user or cause other problems.that can be changed, either be able to choose the level of skill after you select it, or have a toggle on options to show lower level skills, or have a button (like shift) in which if you press it on the spell then it switches to different level or if you press the spell while holding shift then the level selection comes up. etc.
so there are many ways to implement it. UI and quality of life is already very good in this game so dev clearly pays attention to little things so i dont see how this cannot be done.
There are also other factors. Such as that it would entice the player to optimize his play by trying to use the least powerful spell to kill an enemy, but HP is hidden so it would a lot of reloads trying to minimize mana spending.
Players trying to optimize fun out of their game is a major issue.
The other thing is that it's part of the core balancing to scale your HP/MP spending as your HP/MP pool increases.
Otherwise days would be too long. The concept has proven itself in many other dungeon crawling games such as SMT games.