In particular, I would be interested in the opinion of @KlaRa, he seems to have knowledge in this area.
The person addressed thanks for the advance praise.
(as a publicly appointed expert, especially for coating systems, I should be familiar with this topic)
The treatment of the screed surface with potassium water glass is certainly an inexpensive and functional solution—as long as no optical demands are made.
However, applying a coat of paint on top later on for "improvement" does not work that easily.
Potassium water glass seals or fills the pores near the edges, creating an oil- and water-repellent surface; however, due to the different absorption capacities of the screed or concrete, a cloudy light-dark surface appears visually.
And this surface, after the curing of the potassium water glass (purely physically only the contained water as carrier or solvent evaporates), is as hard as a glass surface.
So if you want to apply a coat of paint later, the surface treated with potassium water glass must be well sanded with suitable electrical equipment.
But otherwise, there is nothing to be said against potassium water glass, which, by the way, is also used in the industrial sector as an inexpensive surface protection!
Before any measure, first check whether the screed surface possibly has a slope opposite to the wall surfaces (a spirit level is sufficient as a tool).
-----------------------
Good luck: KlaRa