This is an interesting idea. You already know some things to work on and update for later releases of this (the lag, the music, etc). There doesn't seem to be any particular vibe that this gives off, like a genre of horror or dark vibes. It has zombies but doesn't really "feel" Zombies, which I do understand this is just an early experimental development phase.
I think this is a very engaging way of alternatively helping to reinforce spelling words and that's a pretty thoughtful goal.
There should be a way to break up the zombies, possibly consider how they cluster up into a horde. There would be the one letter I needed but it was bunched in with a horde looking like a novel (it's hard to see the letters in a group of zombies if they happen to group up~ which happens when they chase you). Though you could probably just turn away and look for another zombie that spawned with that letter elsewhere. Not gonna lie, I literally got surrounded by probably more than 20 zombies... lol because I was so super focused on trying to get that letter.
You should consider a raycast / line of sight effect for the bullet trajectory and or even animating the crosshair to signal to the player that they have a target in sight, even though you would think it's obvious. This could help potentially pick out targets from a horde or make the zombie you are targeting, more obvious. You could even change the Letter's color above the Zombie's head for the zombie you are targeting, would actually feel great since you really want to be hunting the letter associated zombie.
The shoot and detonate mechanic is great! It's really fun. I think since you have invested alot of energy and focus into this logic/behavior, you should lean into that and make this effect even more intense. The exploding gore is a nice touch, but you could even do a screen shake or even a camera effect where blood is temporarily projected on the camera (the same way similar to how when you get attacked, you have the on screen damage mark). That effect could be lessened or strengthened depending on the distance from the player where the enemies are detonated. This is a very entertaining element for your game and you can milk that, because playing for that effect is very satisfying. Even though the point of the game is the spelling for points, I am like... yo, I'm gonna blow up this group zombies!! that's fun as hell XD
You should explain and or convey the stamina bar to the player. You would think its obvious but didn't realize I had a stamina bar for some time.
You should consider toning down the volume of the chain lightning effect because that is a loud buzzing especially when there is no music.
Happy developing!! : )