I like the build-a-ship idea! There were definitely a few spots where I made the entire thing a bit too heavy and then ran out of room to safely fuel and power the biscship. There is definitely an element of risk in those early designs. I eventually settled into a bohemian trifecta of dough, engine, and fuel. This allowed me to get nearly 100% success while dumping upgrades into only a few base components. It might be interesting to explore different per-mission constraints or other synergies between different components in order to encourage variety among builds.
Again, really fun idea!