That is really not such a simple question at the moment, one that I also recently faced.
Primarily for cost reasons, I decided against a proper KNX automation system for my little "home automation."
For roller shutters/blinds, smart plugs (e.g. pool pump & salt system), various measurement tasks (e.g. balcony power plant, circulation pump control) and LED strips as well as part of the lighting, I chose Shelly devices, which are based on WLAN.
I already own Philips Hue and will continue to use it in some areas for light control.
I also have Homematic IP and mainly used it in the old house for heating control (window-open detection). As already mentioned, this is not practical for underfloor heating, so I will not continue using it in the new house. (Exception is the vacation home, where the heating is turned down to minimum temperature for weeks).
What I will keep are the measurement plugs for completion detection of the washing machine and dryer as well as the window/door sensors with siren and signal button for the "alarm system." However, I am currently also looking at what integrated magnetic contact switches cost for some windows.
HMIP is reliable but also really expensive.
How I will connect further sensors, e.g. for presence detection, brightness measurement, temperature measurement in the rooms, wind meter, I am not quite sure yet. Aquara is an interesting company here, also from a cost perspective. Currently, the standard components do not yet support Matter; they are working on launching another product line. But whether I want to integrate another system, I also do not know.
Via a higher-level automation system like Home Assistant, for example, you can also connect the individual manufacturers and visualize the whole thing. That then runs on a small server, a Raspberry Pi, or a NAS.
Even though with Matter the radio protocol is compatible between manufacturers, I doubt that all of them will then be visualized in one manufacturer's app.
If you can, I would wait a bit longer until manufacturers have sorted out how and with which devices they will support Matter.
Oh, if you consider Shelly devices, pay attention to a good WLAN network with good coverage throughout the entire house. (Access point on every floor)
Should light switches really always be single? So no double rocker switch for light?
No, they do not need to be single. But make sure that all junction boxes are wired with three conductors, including the neutral wire. That is actually standard nowadays.
Roller shutters:
If you want to automate them with Shelly modules, for example, it is better to use push buttons rather than switches in the installation, which makes control easier afterwards.
Use classic roller shutter motors with cable connection (one wire for up and one for down) instead of radio solutions.
Consider a central roller shutter switch in large rooms or per floor; this makes operation easier.
Other HA points that I have implemented/planned/set:
# I have planned another central high socket in the living room where I will later mount a tablet for HA visualization. The tablet is mounted with a holder over the socket, and a power supply is installed in the socket to charge the tablet.
# A 3-gang light push button in the living room, with which I can call up certain scenes
# The aforementioned central roller shutter push buttons per floor
# In the hallway, I have an HMIP double push button that can also display color states. Here I can see whether the washing machine or dryer is finished, can detect the status of the alarm system, and so on.
# Control of the circulation pump using a Shelly with temperature sensor addon. It detects when hot water is drawn in the supply pipe and switches the pump on.
# Built-in motion sensor in the hallway entrance area for automatic light control