"You can pretty much sound everything out" is simply what I was saying about the orthography. It's partly a result of Spanish's phonological conservatism but mostly a result of the orthographic reforms by the RAE in 01726, 01754, 01815, and 01832. But that isn't even a part of the spoken language at all; it's just a property of the writing system. English written in IPA or Shavian (or my own doomed eccentric proposal,
http://canonical.org/~kragen/alphanumerenglish) would be equally easy to sound out, but it would still be English. If you were talking out loud to someone who had hypothetically learned English after such a spelling reform, you wouldn't be able to tell the difference.
As for irregular verbs, there are quite a lot more than 20; https://howismyspanish.com/all-irregular-spanish-verbs/ lists over 270 irregular verbs in Spanish, but it also says that's "over ⅓ of all Spanish verbs", so it may not be the most reliable source. There's a lot more than five irregular subjunctive verbs, and they aren't all very common verbs; https://en.wikipedia.org/wiki/Spanish_irregular_verbs#Presen... mentions "moler", for example. Other uncommon irregular verbs mentioned in page include "maullar", "erguir", and "embestir". https://web.archive.org/web/20200807095413/https://socratic.... says that the Manual de la Conjugación del Verbo lists 12'290 different Spanish verbs with 63 different models of irregular conjugations; the number of verbs following one of those models must be in the thousands.
By comparison, English has under 200 irregular verbs, according to https://en.wikipedia.org/wiki/English_irregular_verbs.