I enjoyed this, but it's definitely a flawed game. The bullet hell genre is done well, though grinding is perhaps a bit more necessary than I wish it was; I went through the first two levels only to find I did basically no damage in the 3rd level, which was annoying as I was perfectly capable of dodging and attacking but simply didn't have the firepower, making level 3 tedious and leading to very few enemies actually dying and dropping money.
Regarding the actual upgrades, they're okay. Following chess as a theme obviously comes with certain limitations; you can't exactly have AoE attacks or homing bullets without betraying what the helper ships are meant to represent. Unfortunately, this also means that there isn't a ton of strategy to the upgrades, you just buy them when you can and do more damage. There were perhaps 2-3 instances over the whole game where I was able to strategically pick off enemies using the diagonal bullets fired by bishops and queens, but for the most part the utility I got came from these bullets making contact by dumb luck.
With that said, I really did like the way that the helper ships followed you. This made it possible to play more offensively or defensively depending on whether you simply got out of the way of the attack, or whether you got to the right altitude to line up with an enemy before wiggling horizontally to align all of the helpers. The delay on the following mechanism also made it possible to dodge an incoming beam from rooks while still letting your helpers dish out damage, which felt really good to do.
Boss phases are fun, creative, and definitely the highlight of the game. If this were to have any post-game content, I think I'd like some kind of boss rush, maybe with the ability to increase the refresh rate of the game for added difficulty.
My biggest complaint is that the pace of the game is slow. Partly this is due to the current memory leak in Ruffle, but this is an easy fix - just quit and restart the game. The main issue has to do with enemies appearing slowly, with large gaps of time in between waves, the levels themselves being quite long, and the difficulty of on-screen enemies being relatively low. Low difficulty and lack of checkpoints makes it so that most of the difficulty comes from making small mistakes during long, low-risk, high-consequence marathons, which leads to a very frustrating and artificially long game experience when I would have preferred much higher difficulty with less severe setbacks in case of a mistake.
This frustration is exacerbated by some visual issues; tracking your own bullets, the coins that enemies drop, and enemy projectiles is annoying, especially when enemy bullets can sometimes be shown at a lower depth than player bullets, leading to your own bullets hiding incoming fire. Combine this with pink bullets that match the pink background of the final level, the green and white projectiles that match the green and white projectiles that your rooks and queens fire, and the similar saturation of the main layer, background, and foreground elements, it's very easy to run into obstacles not because you made a mistake and got cornered, but because you didn't see the obstacle in the first place.
We've had this conversation at least once before; power creep and visual juice are hallmarks of your games. That's fine, and I've definitely put these features in my games, but one of the first things I do after adding any flashy background and particle effects is to add an option to turn them off. Having player bullets appear at 10% opacity, dimming the background, and disabling loot drops when you already have four queens would be massive quality of life improvements that would require relatively little work, and players that aren't bothered by these things would still get the same experience if they didn't change them in the options menu.
Not much to say about the visuals and sound other than that they're great. The colors are vibrant and varied. The animation is scaled back as you would expect from a Game Jam game, but the small bits of animation that are present look fantastic and there's a consistency among all of the assets that leads to this looking polished.