@ingeborgdot 

I agree with this, also never had an issue with the same SSID and it does make things a lot easier to manage!

Have you enabled both Fast Roaming and AI Roaming?  Ai roaming might help if the clients are not doing it themselves...  Force Disassocation is a thing to be used sparingly as its a total kick off and reconnect (not a roam) therefore the device connection will be terminated.

This only takes effect should the device not action / reply to the roaming command and from my experience... this tends to be older devices (B  G  and early  N)  most new devices roam when told to do so.

I have this enabled on a few sites, rarely if ever gets invoked..   example is I have one client who has a printer (REALLY OLD) that is 2.4ghz WiFi and it sometimes connects to the most random far away APs.  The controller tells it to roam and it doesnt do it, there kicks it off.  Printer reconnects and hopefully to better AP, failing that kick and repeat again.  It wont do it for weeks then, you get a flurry of kicks and reconnects with a few seconds.     Hopefully that helps? 

In terms of channel and power, I would leave this to the controller.   Ultimately most airspace is now congested in some way with other networks.  You will spend too much time fiddling to get it right for yourself, then your neighbour moves something and its all messed again.  Just let the controller do its job automatically, its fairly decent in honesty.

  @ingeborgdot 

Hey

In terms of 2.4ghz only channels 1  6 and 11 don't interfere, therefore despite fing recommending 9/10 using it would interfere with 6 and 11.  Honestly its not going to be any benefit to you setting this channel.

One of the big issues in nearly all WiFi installs is the 2.4 spectrum, unless you live on Mars its gonna be saturated.  You could choose 11 as your channel because its likely the best of a bad bunch (only shared with 1 SSID and not fighting 4) but the reason I recommend to leave 2.4ghz on auto selection is the frequency hopping.    

All ISP routers have a setting that auto changes channel if it detects interference, its a PAIN!!  If you decide to use 11 as its best today, your neighbours may hop to 7 tomorrow and mess up your selection.   If you want my opinion, just let the controller hop you about automatically; its never going to be perfect so expect to see the airspace congested at the best of times, there really is not a lot you can do about this sadly.

5ghz is a different beast, its not so congested and has a lot more airspace to work with so channel sharing isn't a big issue.   Most ISP routers use 36-48 for their 5ghz channels, these are known as the UNNI-1 channels and are the most commonly used worldwide.  Once you go past 52 its into DFS and some regions restrict this, you may also have Radar detection in this range.

Personally I use 52 and 104 for my channels as I like "clean air" and there is no one else on them, i also don't get radar detection.   Try 52 first and should radar be detected, jump up 4 channels and retry

Depending on where you live this is trial and error, however there is nothing wrong with 36-48 for daily use as its got plenty of space in most occasions.