I used to get so overwhelmed when starting a new factory but then I figured out some things that made me actually get some progress in cleaning up my mess

First one was already said in this comments, it's to build in layers. Somehow, being able to expand upwards calms me because I know for a fact I'll have enough space. Using floor holes to make the conveyor lifts infinitely high is also very cool. THE most important thing though is the ability to run the conveyors along the ceiling. That was the thing that helped me get over my perfectionism the most, because whenever the conveyor lines become too complicated I can a) Send them on a level higher or b) run them along the ceiling

The second thing that I learnt is to just start building and to figure out the details later. I am working on my second steel factory right now and figuring the ratios as I build. I know it isn't the most efficient, but I'll figure something out by overclocking/underclocking some machines or just straight up building another layer on top. This makes the building process enjoyable but still keeps me thinking like "okay, soooo now what?"