Did the Last Firmware Update Jack my R6400 from 2016 or could it be dying?

I don't know the exact duration because in internet time it feels like decades, but I think it has  been a few weeks ago in an under 1400 sq. ft. one story home we started to experience more and more frequent dropped connections on different devices, a TON of buffering.

Yada Yada Yada I noticed somthing going on with our different GHz. Now I know there are differences and I am an adult living with an older parent, so this house isn't macked out with the latest and greatest smart devices. I double checked what IS connected and in total we have 10 devices on the router, 3 of those are Kasa Smart Plugs....so not real wifi hungry stuff. The busiest is maybe 2 tvs streaming and a gaming pc being used for business stuff.

I've been kind of watching and changing things around for about a week to see if there was a pattern or a setting I missed somewhere. What I noticed is: I had set the router up so that with the dual bands it could auto choose the least busy band and also auto choose the least busy /best connection channel  for every device but I also gave each device a kind of default primary band. I had most devices grab the 2.4Ghz band and a few key devices default to grab the 5Ghz.

What I have noticed is that most of the time a connection is dropped or is buffering it's on the 2.4Ghz band. Most of the time you can "connect" with the 5Ghz if you need to manually and it will be fine, or it will auto connect to 5Ghz.

I thought great, I've had this thing since 2016, maybe it's dying???

Then I was reading a post that had some snarky responses and I looked at the recent firmware updates (because I DO stay on top of that and rebooting and crap) and it looks like the last one was 12/22/2020. Which would probably line up with when this all started......

so.............does anyone have any thoughts?

Did I miss any vital details?

Should I see if I can install the older firmware?

Do you think maybe it's not related to the firmware at all but something else entirely?

Last I looked this should still be (spec. wise) a great router choice today. I never know what to do about Routers once they aren't a year old any more. I just want to use my wifi and not even think about it.