Old building renovation, house built in 1916, multi-family house converted to single-family home, experiences

I think that is currently very hard to say. Many companies are on short-time work due to material shortages, prices for some materials have risen sharply or they are hardly available. I think it is more like crystal ball gazing at the moment. A lot probably depends on Corona. The situation is very unclear for builders, that is my impression. But that is just a personal impression on my part. When would you want to start?

As far as I can tell, my rough cost estimate is probably too high. Then the question is, what is a rule of thumb for such a project.

You never know whether three times thumb, pi times thumb, or even four times thumb will be the closer estimate to the result. What is certain with an object over a hundred years old is above all that it does no harm if the heading "Surprises" is among the top three cost items.

Where you can save a lot is in gutting. Container in the yard, protective gear on, good tools obtained and full throttle. Disposal and work by the companies is very expensive because there is a lot of work.

Demolition is a ton of work, hard work. The tools are expensive, containers and disposal too. You really have to live nearby. On the weekend, minus Sunday and lunchtime, there's hardly a chance to manage that as a weekend project. Demolition is usually loud and dusty. In this case, probably not so feasible.

And with a real gut renovation, what unplanned things are supposed to happen? If you actually strip the ceilings completely, install a new roof, etc., in the end you still have the roof structure and the load-bearing walls standing. As long as there are no serious defects in these fundamental elements, the chance of nasty surprises is rather limited (?).

Container on the yard, [...] disposal and work by the companies is very expensive,

What does your experience say about the extent to which it is reasonably possible to save by "waste separation," that is 1. what difference do the price levels of waste qualities make for the disposal and 2. are they not offset by the need for several containers and 3. is it rather a theoretical idea from a labor-economics perspective?