I had the exact same issues, reverted to 19.07 and just keeping track in this thread, but will stick it out until there is better news.
I don't see why we would have to upgrade, new stuff is interesting, but not necessary, especially when the newer version has critical issues.
Not bashing the OpenWRT community or anything, this Linksys hardware is just a hard one to support it seems.
That being said, if you have issues with 21.02.X revert to 19.07.X for the time being and your issues will be gone. As long as both still get critical updates, there is no reason not to.