Erik Darling has an interesting query against sys.dm_exec_query_stats to help you determine what might cause your THREADPOOL wait problems:
When THREADPOOL strikes, even the best monitoring tools can have a bunch of blank spots hanging around in them.
If you’re on SQL Server 2016 or better, there are some helpful columns in sys.dm_exec_query_stats.
Erik also has advice to help you out if you are running into these waits.