Clearly you're not using "frontloaded" in the way I'm used to hearing it--that's usually used as a criticism for games that have character building and leveling systems bound to their starting point. 3.5, with it's many problems, is the exact opposite of frontloaded because the choices you make at character creation have only a slight bit of impact on how you would end up at high levels, not counting dumb stuff like Rogue/Fighters being better off starting as Fighters than Rogues because of the extra HP.
In 3E if you havent mapped your character build before you named your character you are doing it wrong. Hilariously wrong in fact, you will more than likely end up with a useless gimp. The system was designed with a lot of bad options that will ruin your character, in fact almost every feat, every skill and every class in the game is a trap choice, this also includes around 90% of the prestiges.
This is the shit that traumatized sawyer so bad that he ended up doing a 360º, and ended up being equally bad for different reasons.
5E has characters that are almost fundamentally complete at character creation because of the archetype system.
This is false, you can make a paladin/wizard/rogue and still end up being good, not as strong as a pure class but fairly flexible and with tons of valid options in any given encounter. You can suddenly at level 10 as a cleric take a level in barbarian and get something out of that choice, your character wasnt ruined, you just opened another venue for the character in exchange for closing a different one.
Clerics have to make a choice of domains at character creation which have class features that they literally will not get for another 5 levels.
Every class is like this, subclasses, which for example as a wizard you pick at level 1, as a fighter at level 3, grant different features until level 17 or so. But a ton of characters can take just the first level of wizard for the level 1 class feature, or the first level of cleric for the first level features and pick a different class to advance. This will not really diminish the character, it will simply open new options at the cost of not opening a different door.
It is true that the first levels of classes tend to be really great compared to the ones that follow, but this is because the ones that follow build up on existing skills, and mechanically theres always more value on specializing than on becoming more flexible, unless you give a real boost to said flexibibility.
Also in true Dnd fashion a rogue/barbarian is a great combo, you can larp conan and be effective!
And in POE every feat/talent/whatever is completely inconsequential
This is not true tho. In PoE your choices do have consequences, you can end up with a shit character thats a pain to play, or you can end up with one that can solo the entire campaign.
The problem with PoEs system does not lay in the importance of the choices in any given build.
so the only leveling mechanism that actually matters is the Wizard picking spells.
This isnt true either. As a wizard you can pick up every spell in the game as you adventure anyway.
Your ignorance is pretty staggering, go back to lurking, come back in 2020.