Performance degredation after upgrade from 12.1 to 18c

I would like to do a reset on my previous thread, dealing with severe performance degradation of some procedures after upgrading from 12.1.0.2 SE2 to 18c SE2. In that thread, we seemed to get wrapped around the axel dealing with the specifics of a particular query. Also seemed to get bogged down in the difficulty of posting in this new forum platform.

Cut to the chase. In regression testing we turned up a procedure that executes in less than a minute in 12.1, and has never completed in 18c, running up to 40 minutes before killing it. If we have this one, then we very well may have more, and we don't know how many or which ones. So I need a broader solution than trying to identify and fix each on a one-by-one basis.