4 weeks of work and only 1 internet connection, is a legal distinction possible?

Well, the issue of intermediary liability is now practically out of the picture. What I would do, however, is of course an appropriate logging of which devices were connected when (regardless of whether wireless or wired).

In an apartment building, one normally takes an appropriate main connection from the provider. The tenant/owner then concludes the contract they want. The tenant then installs their hardware. They have the free choice. If this is not taken into account, it can of course be solved via a router and VLANs. The same applies to the wifi. The problem is that the tenants must not have access to the configuration and you theoretically have access to "their" network. This option actually only makes sense in apartment buildings where only family lives and all parties (at least theoretically) have access to the configuration.

And you theoretically have access to “their” network.

If I am careless as a "normal" internet user, other people also have access to "my" network, otherwise in both cases rather not. The fact that the user ("tenant") uses their own hardware is no difference either, they just connect their router not to a DSL/cable connection but rather to the landlord’s LAN socket. Honestly, I don’t see any real difference here. Thus, the landlord would then also be the ISP at the same time, although in this particular case the necessary expertise seems to be present. Interesting topics might then also be things like SLA, but that can also be arranged.

I would never rent an apartment where I couldn't get my own internet connection. Such a thing is only acceptable in a shared flat or [Studentenwohnheim].

Well, I see it more like "furnished living" or a hotel or something. The great advantage is that you only have to bring one suitcase and can start right away. No need to register water, electricity, or internet. Your data passes through many people's hands anyway, whether it's the landlord or your ISP, it doesn't make a huge difference.

Then you don't have to segment networks but simply separate all clients.