In yet another idea that I have to fix Tier 1 and Tier 2 character, I would apply a surtax to each of these characters, all in the idea that similar XP should yield characters of similar power.
A surtax works like this. As you go up in level, you go into XP debt for gaining that level. For example, getting to level 2 would put your into 1000 xp of debt. You must pay off that debt before you begin accumulating experience.
Tier 1 character would gain 2-3x XP debt per level. Tier 2 would gain 1-2x XP debt per level. Debt is always based on the character level gained, not the class level.
Although the math is a bit awkward, it gives the player a single number to track with a minimum of issues. While in XP debt, the caster may not spend XP on item creation as they are too exhausted from increasing their own power.