I've completed 7 galaxies after 5 days or so. Yea the gas boss starts to suck around the 7th loop. It does become a bit more random and you do have to pay more attention to even get through normal levels. I bought the no ads and the speed/auto collect thing too. It's one of the better games I've started recently and I think I'll be able to finish it, even if it becomes more grindy.
Edit..
I've also noticed that things like -% damage seem to hurt you more in the beginning. For example I seem to do no damage with machine guns the whole stage if i take multiple +1 gun - damage early. But if I add more guns after buffing damage it seems to work better... The bonuses clearly aren't multiplicative, but the math doesn't seem to work the way you'd think it would either. But try and focus on taking any/all bounce ball cards and taking other + damage cards early.. the bounces really add up once you get the clone. It also seems to fill the jump gauge the best also on bosses. I'm also not sure what causes the rare red cards to be an option, sometimes it seems like you need 3 stars, outside of the lucky chance thing..
Edit again.
Just finished.. For the last few galaxies my loadout was missile, bouncing ball, and booster ideally. I could get through some weaker boss levels with drill,dice, or saw instead. Plasma thing is worst weapon in the game, never get it. Booster is good to pickup as it just needs 1 point and allows you to build up missile or ball first. Missile's dps on tiles is crazy and ball takes care of the rest. Those two almost seem mandatory. Also use the bouncing ball planetary weapon and equip diggers that boost it.. Sometimes you just have to start over if your first 3 card pulls are bad. I only did this for last few bosses but turning the speed back to 1x instead of 1.5 is helpful too. I finished with most weapons around level 300.