@Red_Eye80 

Controller not needed for WiFi roaming, this is a standard function of access points and it work fine in standalone.  If the EAPs broadcast the same SSIDs and have the same passwords, clients will roam between them.   Mesh is a red herring and not needed when access points are wired.

VPN - i think so, but someone else can chime in on that

Multiple WAN - The router will not add up the speeds of you rproviders.  They can work alongside each other in load balance, or you can assign them to vlans with policy routing if you want to distribute load that way.

  @GRL Thank you so much for the information. 

Bandwidth: I understand that more WANs do not add up to single user speed. What I want to do, is limit single user speed somehow to make access more fair and equal for everyone. I don't want one client watching a 4k video or maybe downloading a torrent and the other not being able to open a web page. I understand this is any routers main job but I have not been successful so far.

VPN: I would appreciate if someone can shed more light in the matter. Can the ER706W connect as a client to a VPN subscription (like Surfshark) using Wireguard?

WiFi roaming. I hope you are right. Can the clients do fast roaming however or does the signal from one AP/Node need to be really weak so that they switch to the nearest/strongest. I recently got a (wired) mesh setup at home and the difference when moving around the house has been huge, compared to before when I was using traditional APs, all set with the same SSID, password, encryption and not overlapping channels.