In short, the game is fantastic and the graphics and atmosphere.
I will first a) state out what happen and what I am thinking during my gameplay from the beginning to the end, and then b) suggests my opinions. Although my words are not well organized and mixed up with minor and major things, hope it can help.
1a) The game starting page is fantastic, it really tells what this game is about. Instead of using Start, quit these kind of neutral and formal word, the word "Saving Earth" and "Abort Mission" give more emotion to the player and let me get into the story. 1b) The only thing I feel confused at first is the saving earth button that you have mentioned it about the WebGL problems, the trigger area only works near the bottom of the button. (and I am a newbie in Unity and gameUI, I also face similar problem in WebGL platform and don't know how to solve >.<)
2a) After starting, the environment atmosphere is a little bit unmatched comparing to the starting page. 2b) I think the scene maybe a little bit "empty". Since in the starting page, we can see some lighting which provides the futuristic feeling - The animation and spinning of the spaceship, the blue light comes out from their bottom. Also you used a light that splits the earth into one side light and one side dark, this really gives a serious and and solemn atmosphere. However, in the main game scene, I think one of the major thing is the grassland is too empty. the sunset background is full of color likes the starting page space sky, while the grassland is just a simple green platform. It is good that you have made some trees and hill slope at the back. The time in game jam is not much, so I think some of the methods to richer the environment is adding a light and bakemap. I see there are city/buildings at the back, adding light behind so that some shadows will be project on top of us and other grassland area, which can give out more depth of the scene and maintain similar visual compared to the starting page. Another method you can try is duplicating more trees, and distributing them with density and scale variety. Now the trees size in the scene are too similar, I think some is 2x size bigger and a few trees are much more closer or farer to each other may help to enrich the environment. (extra thing is to make grasses to provide extra variety on the ground, but I don't know how can it be done in Unity @@)
3a) The control method is simple, so I can easily get into it, I keep on pressing the right click and keep on clicking the left click. Once I saw an enemy, I just turn on them and shoot. 3b) If there is one more extra weapon that get make the player control and manage one more thing so that the player interest can be maintained more. I think the missile is similar to normal shooting, so it can't increase my gameplay experience much. Maybe changing it to another system, such as a protect shield? So that I need to consider when I should open the shield or don't use it and hence the gameplay can be more interesting.
4a) At first I died after killing 7 aliens, I didn't really turn up and shoot on the sky, in the second try I continue to destroy 22 aliens and I can see the enemy flight. I can expect there will be combination with the two enemies but I just died at this point QQ. 4b) The turning up and down is a little bit too sensitive and it is difficult to turn up in small degree. Also, compare to there is a big difference of speed between the turning of left right and up down.
5a) The ending/losing page is good and gives out the feeling of the doom of the world.
Seems that I typed too much things. Anyway, treat them as light as you like if you disagree the points, I am not a turtle familiar in making game indeed🐢.