My first language is English, but I work at a traditional Japanese company in Tokyo.

I want to become truly fluent in Japanese, without spending too much time getting distracted by the Internet.

As my answers reflect, my favorite Japanese textbook series for beginner-lower intermediate level is the Japanese: The Spoken Language (JSL) series, by Jorden (including Reading Japanese), which, contrary to popular belief, does have an all-Japanese supplement.

Advice to beginners based on FAQs (for a much deeper discussion on these, see my favorite text):

• Textbook > stackexchange
• Japanese is not a translation of English. Translations are merely approximations
• Learn about pitch-accent from day 1
• Kunrei/Nihon-siki romanization is your friend; Hepburn is your enemy
• Check out the Wikipedia page on Second Language Acquisition
• Check out the Refold/Migaku/Mass Immersion Approach/AJATT methods if you intend on making this your main hobby for multiple years
• They're na-nominals, not na-adjectives; and they're adjectivals, not I adjectives. Japanese does not have adjectives
• Negative direct-style verbals are adjectivals; they also accent and conjugate and link up with predicates in all the same regular ways as adjectivals
• Learn the difference between affective (or stative) verbals (e.g. 分かる and できる) and operational (or dynamic) verbals (e.g. 知る and する). Basically, the latter use を and refer to things that are performed by human will, whereas the former use が and are not under our control (hence why affective verbals already include their corresponding potential meanings, but operational verbals have a separate conjugation for that; all potentials are affective verbals).
• Don't dig too deeply into etymology and history of the language until you are an advanced student (though by the time you are an advanced student, you may realize that you are actually only an intermediate student)
• Read textbooks, not websites; seriously
• The rabbit hole is deeper than it looks