So the donut experience turned out to be very useful and made me much more confident with modelling and following experiments. At least I thought that modelling would take me much longer than set up in Unity, but it all happened vise versa.
I had 2 approches in mind for level design in Blender: a solid level buliding and modular geometry.
I started with a solid level like in tutorial here:
I saw benefits in quick carcass of the whole level which I could fill with details afterwards. I all went fine in Blender and I successfully exported the model to Unity. And that’s where problems started. While Blender allowed clipping inside, Unity didn’t, what ended me having just a number of interconnected solid blocks.
But still the experiment didn’t fail entirely as I tried colliders and physics with a 3d object, which went well (and I must note seems a bit more adequate than with 2d colliders). Maybe there is a way to render the object with insides, but by that time I decided that I won’t use this method as it will be hard to make the interior layout with constant need to be inside the object.
So I quickly went for modular geomerty:
I quickly made simple modulars in Blender. Very basic just floo tile, wall tile, doorway and ‘stairs’ (I wasn’t sure how to handle normal stairs, so just made a slope for a quick test).

First I built a simple room and also tested collisions, which worked very good and smooth
Then I’ve developed it in a small interconnected area with flooring. My aim was to test how I can cope with stairs or at least slopes. The complete area looks like this, pretty simple, and very time-efficient with using modulars.
But that’s only camera movement, to test it properly I needed to make a first person character movement, which I never did before (ok, actually everything in this project is something I never did before since it’s a 3d game. So further on just keep in mind that everything was new for me). And though it was tiresome in terms of set up and figuring out the cause of some collision problems, it all went pretty well.
This was the end of my day of experiments which helped me significantly to see where to move forward. So the game will be definitely in 3d, and I can cope with a walking simulator. There are some pecularities on the technical part that I’ll need to keep in mind when exporting and setting up in Unity for the actual level map, but in general I can start thinking through locations and interiors as well as navigation elements for the project.
The next test area is ligting both in Blender and Unity. I tried a little bit of that even this time with a spot light in the first room. But I’ll need some atmospheric and special lighting for my navigation system, which might be easier to do in Blender as I’ve already tried some during my tests with donut. But that’s the topic of further research. So far I’m quite confident in the progress.