I got my blocks running at the 1-2k spm range. At the moment, these are edge-load designs where trains stop at the outside to feed belts weaved to machine inputs. My system only drops below 50 UPS when I bring out a big blueprint, so I think I'm ready to move on to the next level of designing new blocks entirely around train direct insert instead of belts.
Two mods really made this run so much easier when I figured them out, being Constructrons (continued) and Project CyberSyn.
Constructrons really do completely automate blueprint placement entirely but everything in the blueprint must be available to a Constructron Service Station (special version roboport with no range that connects to logistics network). The good news is that you don't need everything in the same geographic location, they just need their own lognet and service station. I find it particularly helpful since my landfill, module, and advanced solar/accumulator construction plants are all a bit aways from my mall. But Constructrons absolutely hang if they can't get something they need so make sure its there. I made a mistake of placing a ton of landfill blueprints to fill in later with woefully inadequate landfill production, so I'd constantly have half my available spiders just waiting for 7k landfill that was building at a very slow rate. But once I overhauled landfill production and storage? Made filling in vast ponds stupid easy. The only other hard limitation on them is a tiny grid size. They can basically fit two portable fusion reactors, four roboports, one exoskeletion, and two Mk2 batteries. They will often get caught on major tile jobs like concrete and landfill waiting to recharge so consider some advanced portable power source mod (like the singularity generator with the Advanced Power Armor mod) if you find yourself really needing more speed.
CyberSyn is exactly the evolution LTN needed to remain useful after train limits came into the picture. Trains being able to immediately queue into the next job without returning to depot truly does eliminate one of the biggest weaknesses of LTN. The CyberSyn depot system goes a step further. Depot stops must be individually named (unlike LTN), but you can allow trains to return to any depot within their network. Refueling is more elegant as well. Trains automatically visit dedicated fuel stations upon reaching below the refueling threshold which eliminates the need to supply at the depot (not a huge issue before with LTN but still just that much easier now). I might be missing a setting, but CyberSyn does seem to favor dispatching a depot train over tasking a train currently "on-mission", but I'm sure I can adjust a few settings to shift that encouragement over to trains somewhere in the midst of fulfilling delivery.