I've escalated my interest level in the Dominican Republic (and to a lesser degree several other interesting Spanish speaking Latin American countries), so while grinding on learning daily 漢字 (Kanji and Traditional Chinese Characters), I'm also going to tackle some Spanish as part of my polyglot agenda.
Do you have any recommendation of a systemic plan?

Best resources you'd recommend like all your helpful recommendations on Japanese?
My tentative plan:
Outline Grammar Points
Because I like to have a reference sheet of outlined grammar points to be aware of, even if I let the "mastery" come through applied examples: I'm going to outline all the grammar points out of "Teach Yourself Spanish" first.
I like the "Teach Yourself" book series so far (especially for Cantonese).
Translation exercises
I'm going to tackle these right away with some cool stuff (eg Conan comics, Spanish language action movie dub-overs) and try to pull my vocabulary and applied grammar learning from that.
I'm sure Lucas88 and others who say Spanish isn't easy at the higher level are right, but from what I've seen so far the "familial" shared roots with many English language words made Spanish seem way easier to get into and "intuit" a lot more of the meaning than would ever be possible in Asian languages.
So rather than worry about stuff like flashcards or anki for vocab, I think I'm going to try to tackle translating fun stuff directly in bits and pieces and pull grammar and a sense of how the language works from that.
(That worked well for me in Japanese and Cantonese too, I just decided to frontload kanji/hanzi so unknown ideograms wouldn't keep tripping me up.)
Background grinding on some language learning apps:
Some people diss apps like Duolingo, but I think they're nice, even if it's not enough to actually teach you to be fully fluent.
I did Ling app on a 1-year subscription because they're one of the few with a Cantonese course, and it was perfectly solid as a basic language learning app with multiple choice questions and so on. I liked it but was kinda done, but forgot to cancel before the annual rebill date so have Ling for another year as of this month, LOL, so might as well use Ling too for Spanish, as well as reviewing my Japanese and Cantonese again.
Personal sentiment: You can't fault the apps or textbooks for coming up with very practical basics, but the totally mundane subject matter is so boring that it's harder to engage with. For that reason I like to actually do my basic grammar outline and look for more memorable materials to engage with, and then test my knowledge with the apps later on, rather than learn from them first....
Spanish has a ton of good materials for translating, by the way:
Conan the Barbarian comics and Spanish translations of the original REH Conan are one.
I also got some books in Spanish on tantric sex (heheh), the kamasutra, and "LA PAREJA MULTIORGÁSMICA" (The multi-orgasmic couple if I got that right) among others.
And I got the Spanish version of David Icke's book "The Biggest Secret" (El Mayor Secreto: El Libro Que Cambiará El Mundo ) about how the NWO is supposedly the work of cabals of hidden genetic bloodlines of reptilian humanoids. (Of course the jews weren't having any of that: "What's that?" they cried, "Reptilians financing both sides of world wars and promoting tyrannical technocratic neo-bolshevist global government while eliminating everyone's freedoms and pushing every imaginable form of sexual perversion to destroy gentile families? No, that would be us the jews, not reptilians." So the ADL and other jewish groups just got right out there and started screeching about antisemitism and sent antifa to threaten Icke and so on, but I'm going to give Icke a read in Spanish and see what his theories are all about.)
I noticed Mexicans also produce a shitload of these frankly pretty f-ing trashy but rather hilarious comics filled with chicks with big butts, LOL.
I think a lot of my macho retro 80s action movie DVDs have a Spanish language track too....
Anyway, I'm happy: I like Spanish because it's a GREAT "gentlemanly masculinity" language. Love how they build in the sexual dimorphism into the language too. Sexual dimorphism is definitely one of my big enthusiasms in life.