Wow Nick! It looks like you hit the nail on the head basically as to what the issue is.
At first the Firewall didn't seem to like that exact format but online it said to try "-p tcp --tcp-flags SYN,ACK,FIN,RST ACK" but that still seemed to have the exact same issue.
Then out of curiosity I decided to comment out both the Up & Down ACKs just to see what happens.
All the sudden the PC doing the downloading seems to have properly moved to Priority 7 (Web)/Priority 6 in the command. Still tons of "dropped packets" but I guess like you said it's to have the server/sender slowdown.
The PC was downloading and the Fire TV Stick was watching content as expected without non-stop buffering.
All I can figure is the download maybe floods the ClearOS PC with so many 64-byte packets that it drove the QoS wild?
Well I finally think I found evidence of the exact issue, but now I just need to actually find a fix.
I found in another post to run:
tc -s -g class show dev ifb0
I noticed I got this response:
Every time the PC does it's download, the "-(1:11) htb prio 1" section, drops packets like crazy.
Well I finally stumbled on what definitely appears to be what causes the problem, but finding the solution is what I need to find next.
It appears that it's something with the download program I am using on the PC, which is downloading a DASH Video over HTTPs. It downloads in fragments and then combines the video.
Anyway, when that downloader is running, the network acts weird.
I tried doing a 1GB test download at full speed, the network acts fine.
It seems like for some reason, the downloading in fragments is what appears to be causing the network issue.
It does it with or without QoS on and it doesn't seem to matter even if you set QoS to an even lower Internet speed than 90% of your Max.
I may have figured out the issue.
I think my ISP must have changed something with the config files of modems or something similar with their systems.
I turned QoS off entirely and did a bunch of speed tests. (To be safe I also verified all coaxial connections to verify nothing was loose).
For years now, my 50/6 would typically test about 52-53/6.5.
For some reason lately, I am lucky for the speed test to reach 51.5 now.
The ClearOS Speedtest came up with 47820 so I did 90% (43038) and 5742 for upload.
It actually appears to be working but I figure I better let time pass before I consider it actually solved.
So far I tested with giving the Fire TV (Wired) a priority of 3 in Downstream.
The weird part is, I checked with jnettop and while I have the download on the PC going, I am not maxing out my up or down.
I generally see 5.75mbit and 2mbit up bring used.
I luckily have a smaller ISP that I almost always get 100% paid for speed.
I also checked CPU and Disk usage just to make sure nothing was wrong there.
I currently have ClearOS 7.9.1 on a 5mbit/6mbit Internet connection that speed tests to about 53/6.5.
I am having some sort of issue where if my one PC downloads something at full speed, all the sudden other devices like my Fire TV Stick don't stop buffering a lot.
I even tried setting QoS to 50/6, leaving the extra my ISP gives out of the equation.
I noticed with jnettop that the PC is doing various connections at once to the download server so I don't know if it's maybe overloading QoS possibly.