Thank you for this tip! It's not yet worked for me (I say 'yet' because I've got multiple libraries but so far I've tried it on just one library).Check For Available Downloads started some downloads in iTunes.
Hope these posts here are helpful to readers!
UPDATE: 'Check For Available Downloads...' still not working for me. Is there some reason to think it's a High Sierra versus Mojave/Catalina thing? I'm using High Sierra...