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So, well, I agree with one point: this design should be used until we need red circuit, so we need to include Green Science production. So I did and actually it works pretty well. The number of assembly machine should probably be increased as well as the number of labs if we start to use this early base for something more complete than just assemble basic stuff but it's not really an issue. The iron output is still too low, but I switched off the long/fast/smart inserters production line by inverting the direction of the splitter after the second inserter assembly machine and I turned all gears inserters away from the belt aside of the yellow belt assembly machine. Now, the gear production is enough and actually I'm probably stacking some. I could easily either stole them for my personal use or directly feed other assembly machines manually in function of what I need (e.g. long arm inserters).
Just an advice for the next step: do not try to bus gears. It's a waste of time and iron, belts (all colors) do not have the throughput to sustain the gears usage for a whole factory. It's way faster to assemble some locally.
PS. I checked. The gears outflow is not enough, due to a lack of iron. But if you shut down the 2nd pair of green circuit production, the iron supply is enough. I probably need more time to be sure of that, but I assume it's a good alternative: with all inserters and belt depend products not assembled anymore, 4 green circuit assembly machines are overkill.
PPS. By changing ~ 5 stone furnace to steel furnace, the production of gears is stable and the assembly machines that make assembling machines can be turned on without consequences on the belt/inserter/science production. The production is however really low due to the lack of efficiency of the inserters. I could increase their number, but I fear it is going to bleed too heavily the gear line, negatively impacting the belt production and thus the green science production.