Based on a Clusterio 60k build by Ralp.
UPDATE: THIS WAS A 0.16 BUILD and no longer makes a full belt in 0.17. Don't use it.
Improvements are 25% fewer inserters, no hiccups with decompressed belts, more compact, nicer wires. 25% fewer inserters is the big one since it impacts UPS. The 3rd assembly machine in each set draws from the back lane of the belt. This reduces competition with machines #1 and #2, and makes all three of them need fewer inserters to get full throughput. This technique may be useful in other belt builds, so keep it in mind!
Beacons are optimized; the #1 and #6 assembly machine in each block only need 7 beacons. The middle 4 use 8 beacons. If you use partial builds for only 1 or 2 belts of circuits, make sure to preserve this to save beacons.