Thanks tnglier, your description of the progression matches with my design goals which is good to hear! Getting straight into the...
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The multiple plots was initially a conceit to the limitations of the device, as you can only have so much going on at once on such a low-powered CPU. But it then went on to become another layer to the strategy. Party Packs are the only 5-ingredient recipe currently in the game and are indeed supposed to be the most challenging automation puzzle. You can see my Party Pack setup in the time-lapse image above. For this one I tried to do it as-much-as-possible in a single plot from scratch, just importing salt. It was very tight, and I'm still refining the setup now as I need more cactus... I used imports more heavily in my other plots.
I don't personally make use of Filters much, but I do find Overflow, Bins & T-Splitters to be common elements in my designs. I'm starting to use cosmetic items more too, but I didn't use them much when playtesting.
Regarding your suggestions.
1. There are some advanced strategies which revolve around replacing the exit of a tunnel with another type of conveyor piece, another tunnel entrance, or even a factory! For this reason, I'll keep it like it is. 2. This is indeed a bug, thanks for reporting! I got so used to using B + Up/Down to resize things that I didn't notice this was broken. Can get a small patch build out once the recent 1.13 firmware has stabilised. 3. Another feature :) either don't input two ingredients in one corner, or if you really need your inputs to be that compact then you will need also to balance your input rates too.