Some things that will tamp down on power levels:
Ritual spells are subject to botch, and the botch chance is equal to the pawns of vis used on the ritual. There was ample discussion on this being part of RAW or not. RAW is contradictory and there is evidence on both sides of the argument. If a 5th magnitude (25th level) ritual spell is always stressful, IMO, then players will take virtues and flaws which will mitigate those botch risks. Further, they will spend XP to master spells, to mitigate botch risks. If you don't let familiars get too powerful (Realms of Power: Magic) then that's also an option.
Tinkering with auras... I had proposed an alternate aura system. This makes a magic aura 3 highly desirable (also see The Fixer's alternate) without needing to automatically push it to +5 with some boons. Additionally, it makes magi significantly weaker in other auras, or even outside of their aura. I like this because it keeps magi very powerful within their own towers, but weaker almost everywhere else, especially cities.
Change some of the mastery rules for spells, specifically multicasting. In Peregrine_Bjornaer's Canaries are Dying saga multicasting must be taken more than once to multicast for each spell, and is no longer treated as a function of the overall master ability. This is much like precise and quick casting are treated. Under RAW, a Ball of Abysmal Flame Mastery 3 with one selection of multicasting allows 4 balls of flame to be thrown. To do that under this house rule, you need to select multicasting each time you want to throw another BoAF.
Enforce requisites for ReCo or ReTe "teleportation" rules. It will make popping to other locations less common, a 15th level Wizard's Leap is within range for almost all magi, whereas the Leap of Homecoming is going to be cast by the Rego specialist more often than anyone else.