There isn't a way to do this via XML, but you can move much quicker through the tiers if you set the "quests_per_tier" attribute value to 1 in quests.xml. (I looked in the code and don't think 0 will work.)
That means you reach Tier 2 quests after a single Tier 1 quest; you reach Tier 3 quests after a single Tier 2 quest (or two Tier 1 quests); and so on.
You will still be offered the relevant "Opening Trade Routes" quest when reaching a new tier.
To do what you want, you'd have to learn C# and do Harmony patches, probably to many methods in the quest-related classes. It would be a lot of work with the potential to introduce a lot of bugs, so I wouldn't recommend it, but it is probably possible.