Your immunity is lost at level 41 you can still trade 1* with immunity at level 40.
If you can get the help of high level hunters try the jan constanz trade (constanz to jan gives less profit bcse of the taxes when you sell in jan). But it takes hours to finish it. You have to take the bird ferry located in spider forest so that you pop up in europe part (in the golem part) to avoid Takla.
If you don't have the help of higher level hunters do the constanz samarkand route and back, it's more benefit than jan to don. Jan Hotan is nice too but the mobs around hotan are a bit high for a level 40 trader alone, especially the white spiders. And the profit Jan Hotan isn't really much higher than constanz samarkand trade.
The Jan Don route isn't worth it, with the taxes in Jan you're better off with gold grinding for 30 minutes (at least in my server where the taxes are max level).
If you like solo trading and have enough time consider perhaps to start a new char that you level up at highest level 40 and delevel it when it reaches level 41 so that you can trade further with immunity and invest in its equipment. The next nice solo trading level starts at 64 (depends a bit of the build); with the fire bull you can do jan to constanz solo (lol, doing the first 5 cities alone is hitting paradise sky level infinite for a trader). Your only concern are highest level thieves just try a not very used route.
Good luck with your trades
