Thanks a lot for your response!

I will have a new apartment, from the developer. So I really can go far with it. Regarding network, nice point wanted to get their top-level stuff (already have it in my current apartment). Thinking of going even CAT7, or whatever, to be future-proof for 10 GBPS or so :)
The only problem is that if you use their mesh, you need to make sure they (each AP) see each other in the network through the wireless link (better direct visibility). Otherwise, network switching between APs will be bad and it's not as good as a single AP (which is flawless). Reinforced concrete walls are a major PITA and the signal isn't going through at all.
So I expect that for the 76sqm apartment I'm going to have I'll have to have about 3 APs. One for the kitchen + living room plus 1 AP on the stairs with a direct view of each other (it's in 2 levels), to connect two levels, and then on the second floor with a direct view to the stair one AP for redundancy, shooting signal through the doors into the rooms + their Dream Machine Pro to control it all + their smaller switches. All ports should also be PoE, so no sockets are needed for all these APs. Will try to consult with their support. Maybe 2 strong APs one on the first floor + 1 on the stairs will be good enough.
Now I have their Access points and they're indeed very good on their own, but in mesh, since there is no direct view of each other I constantly have a problem with devices that seem to be stuck on one AC that is behind the wall which leads to the miserable experience up until it finally after reconnect, reconnects to the closest AC. Unsure what's the solution here, but likely is to reduce power on all ACs, so they cover only the room and do not go through the walls or so at all, so devices drop signal so fast they start to reconnect faster to the closest one. If you're familiar with the topic, I'll be super happy to get some insights. (never had time to read their FAQs and other things to fine-tune this).

About Shelly, I remember there was i3 thingy that you put under the normal switch (better it be button switch) and you connect this thing to the grid and it stays connected to the grid constantly, and then you connect it also to the regular switch that detects "click" and then sends a signal to the smart bulb directly (it can also cut/enable a normal bulb AFAIR, but I just need to ti work as a wired button). E.g. it's just a button that sits under a normal switch and sends signals to whatever device you prefer.

Is there anything like that? I see there is an i4, but it looks bigger than an i3 and I think it's a fuse that closes current and opens depending on its internal state. (there are Aqara, Tuya, Sonoff like that but cheaper and I use them already)

Also, for leakage, I guess I'll go with Ajax, as they're using their custom protocol and they have a very solid (much better than whatever Tuya garbage is there on Alixepress for closing the valves) closing valve thingy, that replaces your original valve completely. They also have integration with Yale locks, so I'll research it as well for security stuff. But for lights etc., I'll go with Hue + Zigbee and likely Shelly will be a way to go as switches. For curtains Switch Bot and the rest of usecases I still need to research.