I don’t have issues with running out of spares for any ships that are on routes that include fleets with lots of spares. I do, however, find I have lots of problems MAKING spares with the default distribution of manufacturing plants on the templates, and have often added extra factories for them or their components as they get low.
Creating new ships doesn’t seem to use up a lot of spares as long as the fleet they’re in is fully “spared” – otherwise, I feel that it just increases the drain which can be really hard to recover from.
I feel like the game is (as is described in the doco) a logistics game primarily. There’s a lot of focus on supply lines and stuff, with very limited power projection. I originally started playing it like Aurora4x, but it quickly gets to the point where you’re building massive stations and increasing cargo fleet sizes to accommodate the need for shipping so much stuff around.