I'm on a 1900acs (v2) and this is killing me as of late. Wifi drops every 30 minutes or so and comes back. Hoping 21.02.2 comes out soon as I am not wanting to risk installing a snapshot build.
EDIT: I'm going to try the 36 channel and see if it helps.
The 21.02.2 will be released from the same 21.02 branch as the daily 21.02 snapshots are built. The stable branch rarely gets commits, so its nature is quite different from the development master.
Trying the 21.02 snapshot is rather small risk in a dual firmware router...
I took your advice and installed the latest snapshot about an hour ago. I'll keep an eye on the network over the next few days and report back.
Well, my 2.4GHz problems kept on going. Some devices had no issues, while a few did have issues.
One really concrete issue, a strange one but consistent, my Harmony Hubs wouldn't connect when on channel 11, but it worked on channel 1. Though my solar panel wireless adapter was the other way around.
I now installed 19.07.8 and no issues of the sort. So there still are issues with the latest snapshots.
19.07.8 is still being maintained in regards to security issues isn't it? Or are there any other real important reasons not to stay on that version? I don't mind not going to 21.02 if there are no real issues.
Following up: I had post-radar detected disconnects on release builds of 19.07.8 and 18.06.9. So it seems this issue's been lurking around a while. I don't know why I didn't run into it before. Maybe I wasn't using DFS channels as aggressively, maybe it is something newly exposed by driver changes to the non-router hardware.
With a lack of upstream support, especially on the binary blob half of the driver, it seems like this isn't getting a fix and should just get a documented workaround - is there anyone with Wiki credentials willing to throw a note in on the wrt3200acm page?
The issue with the Roku was either ephemeral or fixed by the time of the 21.02 branch.
So my final conclusion here is to agree with Mr. Design:
I don't know if there will be further service releases for 19.07.x or not, but you could ask in the following thread if no one answers here: OpenWrt 19.07.8 service release - #40 by Saidiadude1
I've been using my WRT32X's 5GHz on channel 149 since your comment. It's stable for me too.
There will probably be both a new 19.07.x release and also the 21.02.2 release in the next few weeks (early February?).
See the discussion on the developer mailing list:
How is your 2.4Ghz behavior?
If this is the case, it is clear for me. People with affected routers should stay on 19.07.X, there are still apparent issues. You might nog be affected, but buy one of the devices it has issues with and you will be pulling your hair out 
I have been 100% fine since switching and not looking back!
If there are fixes or specific things for me to test, I will gladly do so. I love new stuff haha and I like to help the devs to help us. Easy enough with switching FW partitions.
That one's running solid at channel 6. Never had issues after nbd's fix.
What exact version are you running? Channel 6 is way too crowded here, 1 and 11 are the only usable ones.
Still, even if it works, means something is not right. With 19.07.X all channels work fine here.
OpenWrt 21.02-SNAPSHOT, r16422-e1b79b1dc3
Decided to give all this hard work a chance for WRT1900ACSv2. I am now running the latest OpenWrt SNAPSHOT r18639-f5865452ac / LuCI Master git-22.021.84530-613080f with 5Ghz on channel 36 and mixed WPA2/WPA3 PSK, SAE (CCMP) encryption. I continue to have the following:
echo "0" >> /sys/kernel/debug/ieee80211/phy0/mwlwifi/tx_amsdu
echo "0" >> /sys/kernel/debug/ieee80211/phy1/mwlwifi/tx_amsdu
No significant issues after 4 months (as of May 26). Plan to upgrade to 22.03.0-rc1 for kernel updates.
Is there any reason to use the 21.02-SNAPSHOT builds vs the Master-SNAPSHOT builds?
I use OpenWrt 21.02-SNAPSHOT, r16473-1472a8fa42 on my Linksys WRT1900ACSv2 described by @denisr24 with the mentioned fixes decribed by @User34 except for SQM ? and I only use WPA2 and it seems pretty stable since a few days.
As a side note, I gave the disable Disassociate On Low Acknowledgement a try, but shortly afterwards the wifi was switching randomly between 2.4GHz and 5GHz. So I enabled it again and everything seems to work fine again.
Thank you very much to all the developers and testers out there, you are awesome. 
Maybe the Disassociate On Low Acknowledgement setting was the issue on the snapshots when I tested them. I always have that one disabled, haven't tested it with the setting enabled.
@The others on WRT32X @arinc9 how do you have this setting configured?
One interesting thing about Disassociate On Low Acknowledgement is that Brainslayer over at mwlwifi GitHub repo issues said confidently that the mwlwifi driver does not support the feature at all.
I would assume, considering that, that it could cause potential problems when it is enabled.
OpenWrt documentation states:
This depends on the driver capabilities and may not be available with all drivers.
AFAIK, there are no issues related to specific channels on the 2.4 GHz band.
Is that remark specific to 21.02 builds? Because I have no issues on 19.07.X builds, though I do have that on 21.02 builds.
Makes sense, though it does seem to do something according to reports on using this setting. Maybe the setting is incorrectly named (and does something different from what we all think)?