My first ship to Limit was, small, solar powered, with fuel production on board and supplied by barreled water, SCH and ferric solution.
Power was a huge problem so my next one was going to be run on volatile decomposition (steam power + still using barreled SCH and ferric), but I can't seem to get that to work well.

Maybe I'm dumb, but the "fuel economy" seems to be really bad.
Docking is cool, so I tried capacitor charging and fuel production on SELF, but still struggling to even get enough fuel for one trip.

Fuel production on board and solar power on board are gonna make it practically impossible to get your ship moving, self sustaining designs aren’t viable. They use too much space and add far to much mass to a ship (increasing the cost of moving it, which feedbacks into the cost of supplying it. A brutal cycle).

This works in reverse, reducing mass reduces the supplies needed to move the ship significantly. Ship design in TFMG is based around finding ways to maximise utility, while minimising what you bring, much like real spacecraft. (Drag is still a factor, but less significant. Most ships will still hit high speeds with low thrust, they just take a long time to do it)

figure out exactly what needs to be on your ship, and add no more. Move or remove everything else. As you already discovered with capacitors, docking can help you.

For reference a starter ship can collect about 2500 volatile asteroids on about half a fluid tank of oxidizer and fuel. Try find out how much you can get done on one tank for each.

One thing I will look into is finding some way to better communicate to the player that spaceship design has been changed significantly, and mass is important.

I did eventually try moving things to SELF, but I had like 30 fuel tanks on the ship,
because I failed to realize how much less fuel is needed once the ship is at the target speed.
And you don't even need to slow down on the way back.