Part of our adventures making minimum viable transport ships. This thing barely works, moves slowly, can't fire at many asteroids, only sorta supplies enough energy. It gets where it needs to eventually though and includes the most important space ship feature: Blinky lights
It makes whichever one is lowest in the tanks which means it usually switches each time the recipe completes, and periodically for water
It seems to have an internal bias for more fuel than oxidizer but the engines run near constantly and the imbalance can't cause a backup so I'm not worried about it. I don't know anything about thrust efficiency
It goes around 40-50km/s between Nauvis and Gleba
Well sorry for sending you down this rabbit hole, but thrusters get more efficient the less fluid is in their internal reserves ;)
There's a table on the wiki page detailing how internal fluid reserve impacts thrust and efficiency.
I've omitted the fuel tanks in my newer designs. Tweaking the chemical plant output with modules is an easy way to hit the desired efficiency point. Although it's probably not that easy when you have only one single shared chemical plant.