Regarding textbooks, some people have achieved great success through them, and other people have had a terrible time learning that way. I think it depends on whether or not you’re the kind of person who learns well from textbooks. If you’re able to motivate yourself to study that way, it’s a fine way to start learning. If you find textbooks to be too dry and struggle to motivate yourself to study, you might have better luck looking up grammar points and learning vocab as you go and diving right into trying to read native materials. But immersion can be very frustrating and intimidating early on, which might leave you feeling discouraged and unmotivated.
Personally, I’m going the textbook route with Minna no Nihongo, but am planning on starting to read native materials alongside it once I complete the first book and reach roughly N5 grammar and have a stronger base of vocabulary to work from. Ideally, that’ll make it easier for me to jump into trying to read beginner manga and/or graded readers. I’m also not too concerned with speed. If I’m able to maintain my pace, I should be on track to finish MNN 1 and 2 and reach level 60 in WK about two years from now, which’ll put me at maybe N3 grammar with a knowledge of 2,000 kanji and over 6,000 vocabulary words (everything on WK, plus all of the MNN vocab that doesn’t overlap with WK). At that point, I should be in a pretty good position to read intermediate manga. But I’m someone who works pretty well with textbooks, and that isn’t true of everyone. I also spend a great deal of my free time immersing myself in a form of Japanese media that is largely untranslated lol so I’m pairing this with loads of passive immersion, which is already helping reinforce the things that I’m learning both in my textbook and on WK.
Lots of people claim that this or that route is the fastest or most efficient path to fluency, but honestly I think it’s most important to find a way of learning that works for you, and that you can keep doing on a daily basis without getting burned out. Even if immersion is the fastest way to become fluent (as some claim), that’s ultimately useless if you try it out and burn out early and then quit learning.
I would consider thinking about how you learn best, and maybe try a couple different things if you’re not sure what works for you. You could try reading native materials and see how that goes, or try picking up Genki or Minna no Nihongo (or another textbook if neither of those appeal to you), or try out other resources.
It might also help to find a tutor or an instructor, especially if you want to learn how to speak the language in addition to learning how to read and listen.