So is it your suggestion to avoid those things in order to "improve WiFi connectivity even with internet basic plans", or are you asking for help to improve your "WiFi connectivity"?
*3rd party devices/routers
Keep in mind that, if one does not have a proprietary WiFi router from their plan provider they must connect a 3rd party router in order to utilize WiFi.
*movie apps that require 3 to 6 Mbps internet speed
If one's provider doesn't supply adequate speed they won't be able to stream movies, regardless of a WiFi or LAN connection.
*simultaneous use of wifi from 6 or more devices
Wholeheartedly agree, as the speed will be divided between devices, and not necessarily evenly. And if using the 2.4Ghz band, the WiFi itself could be a choke point, depending on the plan's speed.
* unmindful of radio frequencies (i.e. 2.4ghz/5ghz) or lack of info as its reach among others
With the 5Ghz band this is normally not an issue, though its range is considerably less than the 2.4Ghz band.