I don't do trade tools as they break the game, but any tips on how to design such a route for myself with pen and paper?
all you need is time & patience.
pick a starting system. #1, needs larger stations & large population (decent supply, large landing pads)
find nearby systems that import something from system #1, but also export something in decent numbers. again - large stations, preferably close to main star
ensure the legality of each good you're picking up along the way to the next destination
eg:
hi-tech - buy consumer tech, progen cells, perf enhancers --->
agriculture system - buy tobacco, coffee, tea --->
extraction system - buy bertrandite, indite --->
refinery system - buy gold, silver, palladium --->
industrial system - buy computer components, weapons --->
you can change it up a bit.
industrial to agriculture, terrain enrichers / marine systems etc.
hi-techs pay well for gold, silver, palladium
lots of combined refinery/ extraction places
you might find the odd slave route...
keep away from anarchy systems, unless the station is super close to the star - less than 100ls.
repeat and repeat until you manage to circle back to your starting system