Got a bit further, models are now on the correct position. Demo uses implicit tiling + 1 instanced tile.
https://bertt.github.io/cesium_3dtiles_samples/samples/1.1/trees/
But now there is a strange rotation going on 
I’ve got basically a table with tree point locations (in epsg:4326) and a glTF model (tree.glb), getting it to work correctly in CesiumJS is quite puzzling…
I think I’ll have to add ENU vector in the instance rotation, will experiment next week.