Played it in Chrome on macOS just now. I couldn't get far enough to answer your balance questions, and the reason why is probably the most useful thing I can tell you, so I'll lead with that.
1. Keyboard input didn't register until I clicked inside the game canvas. I hit Enter on the title screen and nothing happened, twice. Once I clicked into the frame it worked immediately. A lot of people will read that as "the game is broken" and close the tab. A one-line "click here to start" prompt on the canvas, or auto-focusing the iframe, would fix it.
2. I then got stuck on the pause screen. It auto-paused (I assume on focus loss, which is good behaviour in itself), but pressing P didn't bring me back — I tried several times and stayed on PAUSE. Possibly the same focus problem biting again, but from the player's side it just reads as being stuck.
3. Two overlay strings collide. On that pause screen, "PUSH P TO RESUME" and "PLAYER ONE READY" render on top of each other and both become hard to read. Easy fix, and it's the first thing I saw when things went wrong, which is the worst possible moment for the UI to look broken.
4. The control list under the canvas is Japanese only, while the in-game title screen is English. Worth mirroring, especially because the Japanese line mentions A/D for movement and Z for fire — the English title screen only tells you arrow keys and Space. You're hiding your own accessibility options from English readers.
Everything I did see looks good, for what that's worth — the title screen with the enemy score table is a genuinely nice touch and immediately signals "this person knows the genre". The formation and the sprite work read clearly at that resolution.
Fix the entry friction and I'll come back and actually answer the round-one difficulty and weapon questions properly — I'd like to, the game clearly deserves a real playthrough.