Saul (Sha'ul) is a Jewish/Hebrew name. Paulos is a Greek name. Even until now, bilingual people who are of a minority culture (compared to where they're living) often have two names, one in their their native (family) language and one in the local majority language, one official, the other unofficial.
This was not limited to Paul, even in that immediate timeline. Levi (Jewish name) was also called Levi (Greek name). There's no reason to believe Paul "changed" his name sheet his conversion. He continued to go by Saul after he became a Christian. He went by his Jewish name among Jewish people, then his Greek name when he travelled across Rome and interacted with Greek-speakers.
I tried out a bunch, including Babbel, Busuu, Language Transfer, Mango, and Memrise. I didn't like them for one reason or another. I finally landed on Lingodeer. It's similar to Duolingo, but it is a paid app. (You can try level 1 of any language for free.)
The regular subscription price is definitely not worth it. It's okay (not great, but not awful) when they do their sales. But I felt okay about paying human workers.
This kind of learning is a great start, but will only get you so far. If your local library has access to Kanopy, look for the Great Courses series on Spanish. I thought that was an excellent series after a little bit of Duolingo.