Maybe use a different method for spawning altogether if it is an unfixable issue.
Could set invisible markers around the environment (one at each corner for example) and each round the ships spawn from these positions.
In-game this could be covered up with a graphic showing a wormhole, ship spawns and is propelled 64 pixels into a direction towards the player with a random add to the angle of + or - 45 degrees.
This whole mechanic inturn tells the player, "ships will always spawn from these 4 points - best to keep away from them in future" and the random angle the ships fly out of each wormhole at least prevents the whole thing feeling too "uniform".
(Part of the reason I'm suggesting this is because multiple times the ships spawned on me/too close to me, inflicting massive damage)
Otherwise love the game, reminds me alot of that astroids game Activision released a few years back :)