If you want to use Tampermonkey from the Chrome extension store, there's no sense in wasting time with Opera. It just won't work. Even if you follow the instructions correctly and Tampermonkey is temporarily unblocked, Opera will decide for you that this isn't good and re-block it some hours later. And since they have successfully been ignoring this support request for the last 5 years, I would not expect anything new. I can understand that--deleting Tampermonkey's ID from the list of blocked extension would literally mean 30 seconds of work, and no one can really ask for that much commitment.
Just use another decent Chromium-based browser. I would suggest Vivaldi (https://vivaldi.com), which is at least as versatile as Opera.