hehe, well, our games are just cousins then. The path I actually took in thinking about this was that DMG2 talked about giving out basically character features as 'treasure'. Now treasure in 4e is firmly fixed to levels, you just get N treasures per level, so in effect it is exactly what my boons are, character features that are given out based on narrative considerations, at a fixed rate per level! Once I decided those were the main advancement mechanic and hit on the reversing the cause/effect relationship between level and boons, then all the feats and PP/ED/Theme, etc became redundant.
In practice you can still give out something like feats, and treasure too. They are the minor boons. Stuff that isn't significant enough to enhance the character's connections to fate and destiny. So a big pile of gold wouldn't advance you in level (though maybe if it was big enough and that was focus of play in that game, then it might actually be a major boon). A potion won't advance you in level. You can learn a new weapon proficiency or language, you won't advance in level then either.
I guess in theory a character could be 'sandbagged' and gain tons of minor boons but not advance. I'd actually say that was kind of a FEATURE of the game though, you can keep playing fairly low level stuff and rarely advance. I mean 6th level could be your highest. As you say though, the 6 boons you got from that COULD be 'epic', there's not really rules in HoML that say "you can't do this till level N". I did put a level on each power, but I wonder if I should even do that. I mean, which boons you can get is PURELY narrative. If the story makes sense with a level 6 PC having an earth-shatteringly strong power, well, why not?
While I'm not a GIANT fan of so-called 'bounded accuracy', I have cut back somewhat on the growth of bonuses, which means that you can probably take on some fairly tough stuff at, say, level 6, certainly legendary tier material (some of the dragons and such things). So I think the upshot is that the game won't break if you hand super powerful stuff to lower level PCs. The math will shift a bit, but not to total wiff-fest so quickly as 4e does. In any case I retained only 20 levels and designate level 20 as the top of the mythic tier of play. That also cuts down on bonuses.
As of now you get a +1 level bonus about every 3 levels, proficiency bonus of +5, max ability bonus of +5 (rare), and maybe another +3 from other sources, tops. So that's something like +26 being pretty much the highest bonus in the game. So if a defense of around 35 is 20th level creatures, then a level 6 guy can ALMOST hit them! Certainly a level 12 guy has some chance. You wouldn't be taking on level + 8 unique monsters, but level + 5 is in the range, and that's a pretty good hunk of the total power curve of this game. I'd say about 50% of all monsters can be 'in play' potentially for high heroic or low legendary characters.
I really may have to show you my RPG at some point, if you're interested. It's very different, but reading your ideas feels a lot like one of the paths I almost went down when designing my game.
In my RPG, you have an Archetype, an Origin (of which race is a part), and a Boon. Your boon is character defining, and the precise rules are currently being reworked to rely less on character creativity. Previously, many players hit a solid wall of analysis paralysis because of how unguided and open ended it was. Anyway, Archeytpe and Origin give you 1/3 each of your skills, and you choose the other 1/3 freely, and both give you 1-2 skills you are especially reliable with.
Rounding things out are Traits, which are like your combination of feats and themes and such, but with point costs, and archetypes providing 1 automatically, and a discount on a handful more.
You gain levels when you've earned 100 character points, which only adds to your pool of Attribute Points.
the attributes exist as pools of spendable points, like Eberron's hero poiints, and don't directly add to skill checks. Instead you use them to reroll flubs, or to activate abilities. Each skill has basic uses that are free, like at-wills, but advanced stuff, and more powerful learned abilities like spells, require 1 or more (usually 1) AP.
So, if you remember the idea folks had to give martial characters more surges, and have their powers cost surges? It's like if you do that, but your attributes and surges are combined into to Reflex, Will, and Fortitude Surges.
I may still use that part as a model for my 4e conversion.
I've gone further than bounded accuracy in my RPG, also. Damage is almost always 1d10, unless you are doing something complex enough to add 1 or more d6. Any such complex action requires AP. HP increases pretty slowly, accordingly, and accuracy vs defense is pretty much within a dice pool range like games like The One Ring. More powerful things are harder to hit, but every creature can be hit, mathematically. The trick is getting around things like damage immunity or resistance, because research/investigation is a pillar of the game.
For my 5.4e, I'm just cutting the numbers way back, and streamlining all the math to follow similar progressions like 5e's prof bonus.