>What I'm struggling with is lack of time and resources.
You are not alone. I'm retired and I struggle with lack of time myself. So, on one hand, don't worry about it because there will never be enough time. On the other hand, don't worry about it because some time spent is better than no time spent, so you already have enough time!
Just as it's good to know your goals as a writer and as a marketer (two different sets of goals there), so you should know your goals in terms of time. Maybe you commit to writing three hours a day, six days a week (just grabbing numbers here). How many hours will you spend in marketing? Remember that "writing" also means research, outlining, reading books about writing, going to a conference (hah!). All of it. So, too, "marketing" means learning how to market as well as time spent actually doing it.
You should also plan to set scope. You already said no Twitter, no Instagram. That's fine. How about a website? A Facebook page for you as a writer? Did you set up a page at Amazon's Author Central? There's plenty to do, and you can take it one step at a time. And, just as you can expect to throw out pages and whole chapters (and sometimes whole novels), so you should expect some time spent on marketing is wasted time. Er, call it learning experiences. Whatever.
I do recommend you do some research on Amazon ads before spending the money. Check out David Gaughran's website and his free books there. Or, look around here at threads on marketing and check out recommendations made there. There are plenty of guides and advice; the trick is finding the one or two that speak to you. Just don't go in blind. It's overwhelming and not likely to do as well as you hoped.
And a final thing. Marketing (a term you get to shape according to your own tastes) really is a different mind-set and process from writing. Some people do their marketing stuff at the end of the day and always do their writing first. For myself, I set aside Wednesdays. Other days are for writing; Wednesdays are for all things relating to marketing (and it's really only two or three hours). You might consider something along those lines; it helps keep one from feeling like the Marketing Monster is ever ready to consume you.