Ok so I must have remembered this mechanic wrong, I could have sworn new traders would be higher tiers, but I guess not. I did go back to my first trader just now and found that they have increased to a higher tier the next day, but I swear the last time I checked existing traders the next day, they were still the same tier, so I stopped checking and just assumed all previous traders would never offer better quests. Thanks for the correction. I'm glad I can now level up to higher tier questing, but it would make more sense if new trade routes and new traders were higher tiered than the previous trader. Maybe they'll change that in the future or explain it better in the game. Maybe add some dialog like "Check back tomorrow for some even tougher jobs!".
Thanks!