Use google to figure out how much real companies spend on marketing vs R&D (e.g., engineers salary). Then you'll know how important marketing is. It's not uncommon to see a tech company spends way more money on marketing than R&D. There must be a reason, right?
Initially, you don't have much money to spend on marketing, but you have time. So how do you allocate your time? It's easier than ever to build a software product. But getting people's attention is getting harder and harder. You have to do work to let people know the existence of your product. Great things rarely happen automatically without you actively doing work. I know engineers like to solve problem in a fully automatic way. But in real business, you need to do manual work.
Every email is a marketing opportunity. It's not like you need to aggressively sell your product/service when replying other people's emails. It's more about providing great customer service. Great customer service is an effective marketing strategy. Genuinely helping your users solve problem (via email), then they'll help you promote your product/service.
It's okay to experiment with paid marketing. When you run a business, money is just a number on spreadsheet :) It's more like resources that you allocate in a strategy game (e.g., age of empires). And paid marketing is not just google/fb ads. There are other forms of paid ads. If you can learn java/python, you are capable enough to learn how to allocate resources (e.g., time, money..) on marketing.
[1] I run listennotes.com