What should I consider when using Wi-Fi?

Green or blue doesn't make any difference.

POE or power outlet either. With POE you save a cable but then you need a POE switch or work with injectors. Both have advantages and disadvantages.

Install one access point per floor and then you are more than equipped.

Green or blue doesn't make the cabbage fat.

would be better than red though ?

POE-Switch

this one is then on the router ?

I would then take the blue position. As for the terrace, I think the WLAN signal from the router will be weak there, so I would put another access point on the ground floor. The question, of course, is where? In [Wohnen/Essen] it would not be great to have an access point on the ceiling. The idea would possibly be an outdoor access point -> [Unifi Accesspoint outdoor] or a [Unifi inWall] in [Wohnen/Essen].

Would take the blue position then.

Maybe a cabinet will go there, which might be large, for bedding etc. = source of disturbance?

unifi Accesspoint outdoor

anyone have experience with it?

Unifi inWall in Wohnen/Essen.

visually almost the same, right?

The inWall hangs on the wall, I find it quite inconspicuous. Advantage, the HD version also has an integrated 4-port switch inside, ideal for TVs, etc.

The inWall is mounted on the wall, I find it quite inconspicuous. Advantage, the HD version also has an integrated 4-port switch, ideal for TVs, etc.

that means this still needs to be placed at ceiling height? Theoretically, it could then go into the partition wall between the living room and dining area, but LAN is going there anyway?