nice game to play.
i think its a limitation to your game to have a max to skill lvls and also the pre requirement to unlock skills.
why would you limit the players options? it just makes your game smaller.
i would suggest unlimited skill values and no unlock requirements.
someone wants 50 magnet ships? why would you stop them?
someone wants to let the game run for 4 hours afk and then come back and instantly buy a high upgrade? why would you prevent that?
think about it. :) the more possible shenanigan's, the more game play the game has.
possibly a good way to limit the player but also increase the game play is to, instead of unlocking, to make it difficult to achieve. maybe a minimal income per round or so. a skill check.