Most virtues aren't really suitable for self-experimentation, because the effects take a lifetime to accumulate. Consider loyalty, a key virtue in Tolkien's books. You can't just A/B test different levels of loyalty so see which has better consequences for you. Loyalty is established over many years, and the benefits come back to you over many decades, so it's not something you can experiment with in your own lifetime.

It's like long-term health. You can't determine whether cigarettes are good or bad for your longevity by any self-experiment. You can only look at historical data on others.

Likewise, deontologists look at other people's lives to decide which virtues seem worth adopting. It's very time-consuming, because people don't reliably self-report their own virtues. The only way to know if someone is loyal or not is by observing, over a big chunk of their life, how they treat people around them. You have to know a lot of people quite well for a long time to make reliable observations on how virtues affect people's lives.

Since you have to adopt virtues fairly young to get the benefit of them, you don't have time for all this observation. The best you can do is to read the books of wise elders and adopt the virtues they agree on.