You build this massive complex factory but then you have to start making adjustments to try and fit new parts in and it just got so complex that in order to proceed I was gonna have to gut entire sections and restart, just lost interest.
Ah. You haven’t learned the most important rule of Factorio.
Don’t “erase” your old factory. Its far more efficient to instead “abandon” it. Space is infinite in Factorio.
“But the biters will attack and destroy the factory” ?? Well, guess what? You don’t care. Automated cleanup. Just abandon it and move somewhere else.
And when you “abandon” a factory, its not a big deal to “undo” your decision, walk back, fix the few broken parts to grab the 400 belts you need for the new factory, and then “re-abandon” the factory. Deciding which parts are abandoned / not-abandoned is a state of flux. You can always reuse / repair the old factory as you spin up your new one. Just do whatever is easiest.
Bonus points: try to abandon your factory after the research of Construction Bots. At this point, you can CTRL-C your good designs into blueprints, and then CTRL-V the “good parts” of your factory over to the new base with very little effort.
But really, the answer is rarely to abandon your working factory. Instead, you use belts to pipe out every useful element of those factories (ie: iron, copper, circuits, gears, steel), and then expand your factory to a new location.