Thanks for sharing this! It’s interesting because I’ve actually been following a lot of that advice without consciously realizing it.
I studied Spanish for three years in high school, did not become very proficient with the language, then dropped it for several years. At some point, I downloaded Duolingo and tried their Spanish course, and I was very rusty 
. Eventually, though, I did enough of it that I got back most of what I had learned in high school.
A couple years ago, after I got interested in pro wrestling, I started actually watching Spanish media (lucha libre shows), and started reading tweets from Mexican wrestlers and such, and I even translated a few articles and interviews because I wanted to know more about LGBTQ wrestlers in Mexico. I’d probably place my skill level around upper beginner/low intermediate, but it was enough for me to start to read with the help of a dictionary.
I kept studying Spanish when I started learning Japanese, and I honestly have never had any problems confusing the two. It definitely helps that they’re very different, and that I already had basic proficiency in Spanish, so I wasn’t trying to learn two different ways of thinking all at once. I’ve actually used Spanish words as mnemonics for some Japanese words, haha!
I started hanging around this forum in early 2021 after I’d been studying Japanese extremely casually for a few months, and I wanted to try out extensive reading after reading about it here, so I tried it in Spanish because my Japanese wasn’t good enough yet. And lo and behold, it worked out great! I read three books in Spanish while I was starting to solidify a steady WK routine and was starting to learn Japanese grammar. Neither language interfered with the other.
I considered trying to use Anki for Spanish, but I didn’t want to drown in SRS reviews, so I held off on it and just looked up words in a dictionary as needed. It was absolutely less efficient than how I’m going about Japanese, but it was also definitely the right decision because it allowed me to study Spanish without adding a bunch of extra study time to my already pretty strict Japanese study schedule.
As far as tools go, I’ve gotten pretty good use out of Duolingo for Spanish (keep in mind that I was mostly relearning grammar and vocab I had already learned previously in school). I started listening to the Duolingo podcast a couple months ago and was very pleasantly surprised to find that I could actually understand 90% of it! I hadn’t realized that I had actually managed to reach an intermediate level in the language. Besides the Duolingo app and podcast, I’ve just been practicing the language by reading books/articles/tweets, and occasionally watching wrestling shows in Spanish. I’ve also tried watching Netflix shows in Spanish with Spanish subtitles, and it has worked out decently well.
It absolutely helps to keep the two languages separate, I think, in terms of tools and such, and to study an easy language and a hard one. One thing that’s nice with Spanish for me is that it has given me a glimpse of where I’ll eventually be with Japanese as long as I keep working at it. In that sense, it’s a great motivator, because it proves that I’m able to do this, and that reading and other practice does indeed eventually translate into listening comprehension.
Honestly, Spanish is much, much more important for my job than Japanese is, so if I was smart about it, I’d be better off prioritizing Spanish and putting Japanese on the back burner. But unfortunately, my main interest is Japanese pro wrestling right now, haha, and I’m not nearly as motivated to immerse in Spanish media (especially since the lucha libre scene has really struggled the entire pandemic), so Japanese is my main focus. But I’ve been trying to actively keep working on my Spanish as well, even though it’s improving much more slowly.
I don’t know if any of these experiences are helpful to you, OP, but this is how it has gone for me. I think if you have a real need or a desire to learn Spanish, you absolutely can study them both at once (though I would definitely get to at least a basic understanding of Japanese, yeah), and I think knowing Spanish would help with Japanese, though I don’t think it would help enough to merit studying Spanish solely to help you learn another language.
Personally, I’ve gotten a lot out of studying both, and I’ve developed a real passion for language learning, period, which has been really beneficial for motivating me to study and immerse in both languages. Still waiting to get brave enough to try conversing with my coworker in Spanish, though