Zapotec sounds like it would be a lot more difficult, but I think really all natural languages are equally difficult, because if it takes children more than about five years to master them, they get simplified, and if it takes children less than about five years to master them, they accrete irregularities, idioms, and metaphorical sense extensions to compress the representation of low-entropy concepts.
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.
I really thought there were only 5 irregular subjunctive verbs: dar, ir, ser, haber, estar, & saber. I've read a couple of novels in Spanish, and it just seems more regular. I guess that's just my opinion. Thanks for teaching me a few new things.