I did some digging, and found
a list of helpful console commands.
I experimented, and the Rest command does force a rest, even if you do not have camping supplies. Like the old games it takes you to a short vid of a campfire while you rest. Then you pop back to the game with full health and spells. So this is one way you can make a mage viable, so long as you do this between every battle. Though that would probably get tedious.
As with the Additem command I used before you first have to open the console, and type in IRoll20s to enter cheat mode first, and then do the same after to disable cheat mode.
I was thinking if you simply modded the spells to be per combat rather than per rest, that would eliminate the craptacularness to playing a mage. But modding Pillars does not look as easy as Bethesda games.
I found some mods that alter gameplay, and tried reverse-engineering to see what the modders did. But so far no luck. Obsidian has not released a game editor like the creation kit. I think people are just using hex editors to open game files. Though I could be wrong. I used Notepadd++, and while I saw a lot of stuff, I was not sure what I was really looking for.
Pillars is using the Unity Engine, so I think I will try looking for ways to edit that in general as a starting point.
This thread from last year talks about a Unity Editor which might be just the thing, but it seems to need some custom extensions to handle Pillars of Eternity.