Wanted: electronically switchable crossover / changeover switch

2. Can this make sense (I know, many alternatives are being discussed here)? Or would it (for professionals) obviously be more reasonable to plan the entire installation with push buttons and impulse relays right away?

Caution: Not a professional! If it's just about building a robust system, it certainly also works with push buttons and relays; this has been tried and tested for years in the hallways of countless apartment buildings. However, I would like to point out that relays can sometimes produce audible "clicking noises." I would pay attention to that. Otherwise, I think Shellys would also be possible over WiFi (I don't really know much about that, you have to like WLAN solutions). Under the keywords "Shelly" and "two-way switching," it seems you can find quite a bit.

Why not just install a few Shellys then?

For (depending on the Shelly) 1-2 reasons. The main reason: The Shellys are installed in the power circuit and switch the lamp. The switches control the Shelly. That means if the Shelly breaks, there is no more light. Most Shellys operate with Wi-Fi. This is great for retrofitting, but anyone who knows radio prefers cables. At least if that is possible with reasonable effort.

Combining "Smart" with toggle switches doesn't sound like a good idea to me. For the "Smart" part, it's not clear anymore what happens when it switches. Depending on the position of other switches, the opposite happens. In addition, a two-way switch setup with 4 switches is expensive and complicated.

Most Shellys work with Wi-Fi. That's great for retrofitting, but anyone familiar with radio technology opts for cable. At least if that's possible with reasonable effort.

Then you should use KNX directly and not a (complex) DIY solution.

Combining "Smart" with crossover switches doesn't sound like a good idea to me.
For the "Smart" part, it is no longer clear what happens when it switches. Depending on the position of other switches, the opposite occurs.
In addition, a two-way switching with 4 switches is expensive and complex.

Yes, that's true. With such a circuit, the "smart" part could not actually turn the light on or off, but only toggle it. Whether it is dark and therefore the light should be toggled (e.g., with motion detection) is probably easy to do. The opposite might be more difficult, because when it is bright, I don’t yet know whether the light is on or not. So here I would need a "lamp is switched" detection.

So in our bathroom, for the main lighting, we have a normal light switch from the hallway. That was the plan.

But it was too bright for us. That's why I put a Hue E27 in the ceiling light and took an E-switch from the Hue program above the hallway switch, with which I can control the color temperature and the brightness. The light switch is always "ON". And next to the mirror cabinet, I have an identical E-switch again, which controls the same light. So you can readjust there. You could also integrate a PIR switch, etc.

And if there is a two-way switch instead of a simple switch, you set it once to "on" and then only switch with the Hue switch. If the switch is broken, you switch it off once and then on again, and it works like a normal light with two-way switching.

If needed, the conventional switches could also simply be converted to sockets (i.e., let the wiring pass through and install a Schuko socket in the box).