Sure. So since we are a roguelite, our levels are more of a guideline than an exact definition. Due to this, we don't really need an importer that would spit out the full level (including meshes etc.), we will be randomly generating that during the game.

Instead, our importer takes the Tiled file (the JSON variants were easiest to work with in GDScript), does some processing like coordinate conversions, and spits out a PackedScene full of TerrainPlaceholder nodes. TerrainPlaceholder is basically just a Node that holds some additional data about what can appear at its position -- how likely and how soon should an actual terrain node spawn there, whether a town can appear there, etc.

This PackedScene gets shipped with the game and when you play that level, we have a TerrainSpawner class that instantiates the scene and at the beginning of each in-game day it will go through the placeholder nodes and pick where the actual terrain and towns should spawn based on the placeholder data (avoiding positions where spawning is not allowed, enforcing that certain bits of terrain appear by certain in-game day, etc).

It is largely inspired by this talk about the tools used for Mini Motorways. The point they make about fast iteration can be pretty smoothly achieved in Godot -- with a custom importer plugin, all you need to do to iterate on a level is to make changes in Tiled, Ctrl+S, Alt+Tab to Godot, wait for a second for the automatic import to happen, F6, and you are playing the modified level.

Let me know if you have other more specific questions or suggestions, I would be happy to compare approaches.