2.4g and 5g with multiple extenders

I have an AC1900 Nighthawk on the main level of my house with an AC1900 Extender in my basement and had the 2.4g and 5g bands separate.  That was working fine, but on the other end of our house we recently installed some Nest equipment, another Alexa, and two TVs with streaming.  We noticed that after those were installed, sometimes Nest or the TV would buffer for around 30 seconds but would eventually connect.  I thought a good solution would be to purchase a new extender (the AC3000) and I installed it close to the newer devices on the other side of the house.  When I did that, I went online and read several articles about naming my SSID the same for all bands vs keeping them separate.  I decided to combine them into one SSID so now with the three devices, the buffering has decreased somewhat but I'm not sure if I'm doing the right thing so I had some questions for you network experts.

 

1.  When I have one SSID, how does the overall system assign devices?  Would it matter if for some reason one of the devices on the other end of the house somehow connected to the main router?  What if that happens and the signal later degrades?  Will it somehow shift over to the nearest extender?  I assume I want the devices on the other side of the house to attach to the extender, right?  Or does that matter?

 

2.  Is there an advantage to creating six separate SSIDs (2G Main, 5G Main, 2G OtherEnd, 5G OtherEnd, 2G Basement, 5G Basement) and forcing the stationary devices in each respective area like Alexa, TV, Bose, XBOX, etc to connect to a single SSID while just advising visitors to attach to whatever network is the strongest when they select it?

 

I've googled all over the place but I haven't found answers to my specific questions.  I apologize for being unaware, so thanks for the help.