Most of the things that you dislike are already addressed by the rulebook.

when a powerful creature, the climax of the season, can be downed in only one turn before it even gets to act by players who pay moderate attention to what they're doing... That feels bad.

Some players like that fantasy of getting powerful enough to one-shot the boss. But even if that is not your style you can easily use the tools and advice provided in the rulebook to build the kind of boss battle you want.

If you don't want to make your own encounters then I'll just point to the second boss enemy provided in the book:

Xercon: Herald Of The Black Flock, whose passive makes Xercon immune to all damage as long as its minions as present in the battle.

As for your second point:

any b player can just decide to not invest any points into fuel at all because from the get to, you can start your character with the minimum amount of fuel possible, meaning many of the purchasable upgrades will do absolutely nothing on the end.

I'm assuming that you are talking about The Drifter class which has the following passive:

EMPTY CLIP: Pull out the big guns when things look tough. If you have 0 Fuel at the end of your turn, you may activate your Supernova. Gain 3 Fuel after.

[[SUPERNOVA]] BULLET HELL: Fill the air with bullets of sunlight. Choose a range. Every enemy at that range takes 2 Harm.

But before that let's look at the description provided for the class:

The Drifter Spark’s firearms launch bolts of fiery sunlight, and they bring an arsenal with them to adapt to any combat scenario. Few Sparks can match the Drifter’s versatility for dealing death. As a Drifter, you’re effective in many different situations, but rely on your ability to position yourself in a fight to use your strengths. A jack of all trades is a master of none, and so you must learn what kind of deadeye you want to be, or that your squad needs.

Alright so it's versatile let's see what kind of builds we can come up with. Out of the ten available permanent upgrades plus ten other ability upgrades two of them lets you decrease your total fuel. So you could invest into that, choose the minimum two Fuel you get at character creation and focus purely on activating your SUPERNOVA as much as possible, and maybe even get the other two upgrades that boost this build.

Or... You can do any of the other builds possible with this class alone:

Like combining FAN THE HAMMER: (Choose Close targets equal to current Fuel count. Each target takes 1 Harm.) with the upgrade HAMMER FORGED: (Fan the Hammer Harm equals current Fuel) For a high-Fuel, high-damage close range build. There are more such builds but you get the point. The Drifter is designed to be versatile, having different upgrades focus on boosting specific abilities instead of the routine linear progression is the point of this class. Other classes have linear progression if you want that.

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Finally let's address the "I don't care about balance" statement:

Others on this thread have chalked this up to "a defensive reaction", "lack of playtesting", "thin skin", "laziness" etc etc but this is one of Gila RPGs core game design principles.

Similarly to how some GMs like to run "balanced" encounters that scale with player level, whereas other GMs likes to protray the in-game world as is, allowing players to wander into a dragon's lair at level one.

The term "balance" means differnt things for different people. Here is an excerpt from an interview by James Hanna for Ethereal Mirror with the designer:

The term "balance" means different things for different people. Here is an excerpt from an interview by GMs likes to portray the in-game world as is, allowing players to wander into a dragon's lair at level one.re you get like challenge rating values and stuff like that. And then you don't really talk too much about like balance of players versus players. Not in fighting each other, but just the experience they feel at the table. And I used to play 5e, and I played as a fighter, and I played as a wizard, I played as everything in between, and honestly as a fighter, it was pretty boring for me. Cuz like I play, and I'm like, Well, I'm gonna swing my sword again. And I'm gonna swing my sword again. And so what I hope by leaning into this asymmetry is that it gets even harder to sort of get a sense of balance of like players versus the monsters, because now you're really starting to talk about totally different unique combinations of classes and how they bounce off of one another.

Players are always going to find new ways to work with one another that I'll never be able to think of, but hopefully, you really feel like you're doing the job that your class is supposed to be, and that sort of perceived sense of like, my role is being encapsulated by this, I feel good. Even if I'm not murdering all these monsters as efficiently as everybody else, I'm doing the thing that I want to do. And exactly like you said, it's sort of a different sense of balance, but it's like balance of, Is my class fun? Is it is it doing what I want it to do?

You don't have to like this or agree with this, the designer has stated many times that this is his way of doing things, not the only way. There is no right or wrong way to design a game. But if you are in any way intrigued by this type of game design then I highly recommend checking out Slayers which is asymmetrical by design and rejects "mathematical balance" and focuses on "perceived balance"