[matsz]

If you live in a small/medium apartment then 6E is quite good. I've been using one of those "enterprise" tri-band APs for around a year and my experience has been also amazing, with just one AP in the entire flat.

Enabling all bands should allow your device to just drop to 5GHz/2.4GHz when needed. This has been seamless for me.

[Yizahi]

Depends on the wall construction. Bricks or some AAC will be fine. Steel reinforced concrete panels - and you might as well call it Faraday cage.