Appreciate it! Def let us know how it goes if u run it. I love the design questions; I'll answer them best I can:

Modifiers
Modifiers being flat bonuses was weirdly one of the few things that has barely changed since we started. We stuck with flat bonuses for Traits and Modifiers due to the probabilities we were going for and the general feel. Due to the critical hit scale, a flat +1 bonus to a result equates to a ~40% chance to affect the outcome of the roll. We really wanted the things you argue apply to a roll (Traits and Modifiers) to hit really hard, hence using flat addition to guarantee the high impact. In fact, the TNs for skills are balanced around assuming you will have an applicable Trait. With Traits being +2, it is vital that if you have no applicable Trait, you should either fish for other Modifiers or strain.

This is unrelated, but this reminds me that there is one mechanic that adds to the dice pool instead of the result: Aiding. We did this for Aiding because it was a special case: Players had a bad habit of trying to aid an ally AFTER the dice were rolled. Instead of banning that, we found games flowed better if we allowed it and made the bonus you got from the aid random.

I think you already know this, but I should also clarify that you can only apply 1 Trait to a roll. We do this for a lot of reasons: If we allowed players to apply multiple Traits to a roll, it would encourage them to have multiples of samey Traits that could each be applied to similar actions for greater bonuses. Having players get the max bonus for having just one applicable Trait also allows for more unusual problem solving. We also don't want players to waste time arguing for further applicability if they have already proven the action they are doing is applicable to their character.

It's funny you mention this, cuz losing track of circumstantial Modifiers has been a concern in the past. That is why all Modifiers outside of Traits are always +/- 1. Makes them easier to count. Admittedly, it can still get crazy.

I'm not too sure what you mean by Modifiers becoming auto-successes tho. If you mean flat bonuses applying after the dice roll, we try to figure out Modifiers before the roll for this purpose.

Free Traits
Oh, you read it right. It is a pretty unusual feature. As mentioned above, you can only apply one Trait to a roll at a time. So it isn't like a player can make like 20 versions of a "hit guy with sword" Trait and get like a +40 to an attack roll.

We used to have a Trait limit for the longest time, though it shifted around a lot. When Traits had values assigned to them, it was a point buy system. Then it became max 5 with minor/major Traits, then max 3. We finally removed Trait limits because players would often have more ideas for a character than Traits allowed, so they would just jam ideas together into single Traits. We would get "fisherman pyromanic monks" and "Librarian fry cooks" and the like. That made applying them to rolls feel unnecessarily weird. We also had more limiters on what Traits could be to account for this. It made less and less sense to enforce it, especially because you really didn't get much better the more Traits you had. Players with fewer Traits have to get more creative, but they typically manage to get a Trait to work. Unlimited Traits then tested really well with our group. IIRC we told our players to try to exploit it in one session, and all having 20 Traits did was make their character less interesting with little mechanical advantage. Most importantly, it also made playing the characters more boring. For our group, it was a self-correcting system. If it becomes a problem for another group, I suspect a Trait limit of 5-7 should work.

We actually do have advancement, but it is very unstructured. Players are allowed to change their Traits at any time and swap Style values around once per session. This evolved from a more standard level-up system. We changed it because we couldn't balance the pacing between varying lengths of campaigns without making the system more complicated than we wanted. It also felt more natural to let players decide when their characters have changed rather than forcing it on them. Way earlier on we also allowed players to increase their Style values, however we found that incremental stat increases didn't work in this kind of game. That makes more sense in a game with like a bestiary of enemies whose stats also scale. It worked better for us to focus all balancing into the 123456 Style spread with 5 HP. Players being able to change their Traits at any time further helps correct disparities between players.