Warning: Even my larger designs normally fall well short of 100k mass, because I don't have a lot of time to spend in-game, so I'm probably not the target for your post.
1. Using advanced build mode (not placing not block at a time), I build up the exterior to look nice, which adds to my motivation. (Sometimes I have to fly out and just enjoy a look at the shell of the ship to appreciate it again.) I try to maximize my use of shapes that take advantage of advanced build mode (avoiding lots of small bits and pieces that would require laying down single blocks, so that includes avoiding a lot of angled corners that are 45 degrees in two different axes).
2. I will continue to design internal layouts as I go, to help keep the inside orderly. I will redesign not only RP space, but system layout. Working on refreshing designs helps keeps me interested in the project.
3. With my latest project, I have taken to using docked "rooms" to populate my RP space. With this method, I can put in multiple similar rooms by building the room only once with a docking block, then spawning in multiple copies and docking them all to the host entity where I want them. (I'm still learning how to use speed controllers to minimize lag.)
Ultimately, if your heart isn't in building, building will never be a thrill for you. You should probably join a faction as a pilot (versus joining as an engineer.) That means you'll probably want to be a member of a faction that already has at least one passionate engineer, so you can fly, test, and otherwise use his builds. Ships are instantly more interesting if they are built by a friend (faction co-member) instead of a stranger (the rest of community content).
