Very solid game. Played through to Nightmare +4, no health lost.
Some graphical flickering (Boss disappears with lots of bullets - flickering on damage?) and sound distortion at higher levels. Significant slow down in the nightmare levels - might need vectorisation or other optimisations to handle the quantity of objects. Ship power improvements might be an option - I ended up upgrading the cannon speed and just using the recoil to move around.
That said - probably the best game I have tried so far. Simple mechanics and idea but executed well. Despite balance being a bit too much in favour of the player, it was an enjoyable experience and kept me playing much longer than I expected. Nice work.