Your 60/40 number is what made me raise an eyebrow. Forty percent of the PHB is roughly another 120 pages. Even if we're only talking the stripped down Basic Rules, that's still almost 50 more pages of pure rules. That's a lot of new rules to remember for players and DMs. Personally, if I bought a system that I felt was lacking nearly half of the rules I needed, I'd be largely disappointed and probably looking elsewhere.
Of course, you know best how to run your game with your players, so I'm glad the Courtney Campbell book provided the tools you need. I may have to take a deeper look at his blog and see if there's anything I might find useful.
"40%" is a fraction of tabletime spent, not a page-count of anything. In my current game, my players spend maybe 10-15% of their time on Civilization-like activities such as trade, recruiting colonists, and running for mayor. But it doesn't take 10-15% of the PHB in written rules to make that work. So far I've given them only a handful of rules, which could probably be written on a single page. It doesn't take a lot of pagecount to take up table time.
So if you wanted a campaign that was going to be 40% about planning trade routes, you could probably get by with a map labelled with monster frequencies and distances as well as trade supply and demand nexuses, some simple rules for using the map as a hidden Markov model to generate a similar map for the players to see, five pages of rules on wilderness navigation/hazards (much like in Out of the Abyss, and in fact perhaps those rules could be adapted), and another five pages on trade using the numbers from your map.
E.g. Thule really wants slaves, it's a +5 for selling slaves, and Rwena is having a civil war, it's got an abundance of them and is +15 for buying slaves; but to get from one to the other you have to cross the Deadly Mirk, which is a danger area VIII two hundred leagues across. Getting across the Deadly Mirk alive as a lone adventurer is one thing, but getting two hundred slaves across the Deadly Mirk will attract all kinds of deadly attention, giving x4 on random encounter checks.
You could hack something like together today (as demonstrated by the fact that I just did) but it would need considerable polishing before I'd want to give it to players. You'd need to think through the implications of the system and whether it rewards behavior that feels "realistic" for traders. For example, will PCs using these rules naturally begin to consider tradeoffs between warships and merchant vessels? Would they
ever sacrifice another six catapults for higher profits on trade? If not then the rules aren't working right, they're just murderhobo rules like regular 5E and not merchant rules like this campaign needs.
Ideally, once all the rules have been read through once, the "mercantile cheat sheet" can be written all on a single piece of paper. Together with a map, that is all the players will need during play.
