Interresting project Dune.
In many aspect it's remind me Svalbard in summer.
You put almost every ecosystem we could find in this archipelago between June and September.
I don't have a large experience about tundra and post-glacial environment but I've some comments according to what i saw maybe it can help you.
* threes are rare, tiny or non existants ! but there's an incredible low level life in tundra (mostly moss compose by different vegetals species)
This moss parts are really really thick and compact. It's like a 60 cm thick mattress full of water. glaciaal-test-3E-crop-2.jpg is very pertinent about this. I've saw many water hole like this (not sure about the sand around it ).
* the main problem to me is how you spread all those different ecosystem. You switch tOo quickly from one to another in a random way.
Every ecosystem are covering a much bigger area before switching to an another one but changes to an ecosystem to an another one are quick like you did. There's almost no in-between or blend.
The river in the second picture is exactly what i saw in Svalbard. This kind of ecosystem is present near sea shore for 1 or 2 km.
Further in land the river get finer and more "mono-path" and creates some kind of small canyon (10 meter depth) with less sand and more rocks. More you go in land more the vegetal part came closer from water.
*Water in sandy part is very yellow brown and non transparent. Water in tundra is crystal clear. River Water is clear further in land.
*you could add an new echosystem between tundra and sandy parts in some place : voronoi cracked clay. Like in deserts. But the clay is so "strong" water slip on it, it's almost like a non Newtonian fluid. A friend of mine loose his boots in this shit !!!! If you run it's solid if you stop or walk too slow you gonna be trapped.
*Rocks should be visible only on sandy rocky part. They are present in green parts but they're should be fully covered by moss. As i say the moss is very very thick. You can run and jump face on ground without any damage. It's "bouncy"and confortable.
Hope it help.
Anyway you render are already very interesting.