I wanted to quote what you said part by part, but I didn't want to take even more of your time so i'll quote only this.
Nobody should ever apologize for helping others, and I have to say, these are precious and helpful informations.
I am not quite set on everything yet, not even the species, or even if I feel like spending 200€ on Xaxim (that thing gets pricy!).
I'm even tempted to convert my 180g channa tank into a terra just because I need something new, and I'm kind of stuck with weight since I live in an appartment, and I know I won't get bigger until it's time to buy my own house.
But dedicating that big tank for a single species might be what keeps me at bay with just the smaller terra. Everyone has to start somewhere, but I find a beautifuly decorated terra to be easier to look at than a fish tank, just because it looks so natural when well done.
I'll keep thinking it through, and again, thank you for all the precious informations.![]()
The Xaxim is definitely worth the price, granted it is a big expense.
moss and ferns will often pop up after a while given enough light and moisture.
I honestly find vivaria easier to look at as well. Plus there's far less algae to worry about

Mixed species tanks are the most interesting to me, and I recommend you research each species individually before doing so,
Many reptile keepers are paranoid about mixed species tanks, its more of a random phobia than anything rational.
And where the heck are you seeing 3 foot mertens? I never see any get over two foot,and most were smaller.
Random phobia? Novel pathogens are a legitimate problem, and seamingly small animals can eat surprisingly large meals. I've seen pictures of Phyllobates terribilis eating geckos. Having a cautious approach (or "phobia") is much better than throwing seamingly similar animals from around the world together because they are colourful.
Including the tail, V.mertensi can easily be ~3 feet. Most papers note them at around 1m (~3.28ft). Granted, their svl is half of that.
Perhaps I should've put "a potentially 3ft lizard"; my mistake. Still not a whole lot of room for a semi-aquatic monitor.