Great! I haven't played William's Defender so I looked it up to see where the inspiration came from. Looks like an interesting game. I like how you took inspiration from a single player game and tweaked it into it's own game with a multiplayer experience, bases and tug-of-war gameplay. Well done!
"Aim is currently part of ship movement - while moving up the bullets keep upward moment, vice versa for down, they fly straight when you fly straight. Was this clear or did it seem like the bullets were firing randomly? I think I can fit this info on the join screen."
It took a while to figure out and later I read about it in the HowToPlay. I think it would be good to have an instruction in-game or make it intuitive by tilting the ships in the shooting angle when going up or down.
"The game starts fairly slow and then picks up every 30 seconds until it's frantic and doesn't get any faster at the 2:30 mark but I am considering lowering the top speed a bit."
Ah, so that's what was happening! I noticed the game was slower in the beginning but I wasn't sure if it was a game speed glitch or something intentional. I suggest to add a line about it in the HowToPlay.
"I am also planning to make the bomber and swarmer waves spawn faster. They currently start at 20 and 30 seconds, new speed will be 15 and 20 because I feel like the lander factories are way more valuable at 10 seconds per wave (6 seconds at max game speed). What do you think?"
Not sure what to suggest here. If the spawn time is lower then the bases will be more valuable but also harder to steal with more enemies close to the bases. I guess it's something that needs to be iterated on. Game balance is often a tricky thing to "get right". Maybe find a good setting that is fun to play and then additionally expose the values in a property file or modifiers so people can adjust them if they want to tweak something.
BTW how is the difficulty increased from level to level in solo play? I feel like it ramps up quite quickly for me but I'm not sure how it's done. Do the computer controlled ships get better?
Good stuff! Keep up the good work!