I like the idea of trading Cousins, even if it's for a mid-round pick. Alex Smith a few years back got a 3rd + a starting CB, so if you can do something even remotely in that ballpark I'd pull the trigger. Kirk is fine, however, at a $45m cap hit, they'd have to cut multiple quality starting (albeit aging) players (Thielen, Harrison Smith, Michael Pierce, etc.) just to get below the cap. He's not worth doing that.
Trade Kirk, clear $35m in cap space (after eating the $10m), & try to find a bridge type of QB. I would be fine with Jameis or Mariota. A lot of analytical-type QB numbers don't see too much of a difference between them or Kirk anyways, & they'd likely be available at about 1/4 of the cost.
If one of them remotely works out, you have a guy for a few years. If they don't work out, you're likely picking in the top 10 & can take your shot at a top tier QB in 2023 (CJ Stroud!!!). Even if one of those types of guys does pan out, you can still be aggressive & try to trade up for a top guy.
This franchise just needs to get a franchise changing QB - whatever it takes - just please finally find one.