I've been perusing Mutants & Masterminds, but I think there's too much going on there for me to bother with....
Ideally my super hero game would be based on something like true20,
Given the second part, you should reconsider the first part. Because True20 is a clunky version of M&M (I love True20 but I'm just saying what I see).
M&M 2E is one of the smoothest running RPGs I've ever come across. It gets jarring in character creation, with some players coming to the game with preconceptions about mechanics (usually from other systems) or a stubborn unwillingness to try and learn the rules, but is actually pretty simple and fun in play.
Actions* are resolved with d20 + Stuff to hit, and d20 + Stuff to save and resist.
As a GM you only need to learn the rules in use (this goes double for powers). It's nice if your system mastery includes all the powers because you can help PCs use unusual power-choices for the flavor they want, but it ain't necessary. So, learn attack / defense (fairly simple, and I'm sure you're almost there), the save system (really simple, especially for someone that knows True 20), Knockback (damage minus knockback modifier; look at chart on page 70, that's how far people fly), and any powers that get used a lot (use bookmarks until you've learned them, no rush).
Scaling saves are always on the -5 model (-5 is worse, -10 is really worse, -15 is worst). Not all saves scale.
The archetypes in the back of the book are great. I ran some (18) Triad thugs the other night, who had no character sheets; all minions, a +3 attack with light pistols (+3 damage), a 13 Defense with a +2 Toughness (+1 flat-footed), and Fort +X, Ref +3, Will +1. I flipped to the back of the book and noticed that the PL 3 Ninja archetype lined up in most places, so I just used those stats when I didn't already know what I wanted them to have.
Much fun and shooting of PCs was had. Good times.
Most of the book needs to get referenced during character building; outside of that, just look up some mechanic you don't use often (Fearsome Presence, Mind Reading, and the Conditions list and the end of the yellow chapter, for me) when it comes up.
M&M has a lot of crunch, used to create characters. It
plays like a rules-light RPG, which is the only way to
feel like a superhero game.
Good luck.
*Grapple, physical or mental, is an opposed check. There are a few other opposed checks as well (power nullification, mind reading, power-countering checks). These are fairly rare and very simple; ties go to the biggest bonus. Some powers are Perception Range, or Area Effects, and don't require attack rolls.