I agree 100% with all of your post. Gray is my guy out of that group as well as long as his decrease in K/9 the past few years isn't a result of a decline in stuff. If his stuff is the same, you sign Gray, get him out of Colorado, put him with Petey, and reap the rewards. The guy is an ace if he puts it together and the stuff is still the same.
Gray + Thor + Max Max (Or Ray, Rodon, Gaus) is my ideal pitching offseason. Thor would only be a 1Y deal so he's off the books after 2022. Gray, like you mentioned, I'd try and lock up for 3Y cheap AAV. Mad Max is probably getting 3Y high AAV and he's ageless, so I would do that without hesitation.