It's a hard topic. My team mostly does experiments, A/B tests on new merchants to see what nudges leads to better results overall. The reality is we have a lot of ideas and we're trying to get data to figure out those answers. But we don't truly
know yet.
The hard part (as far as I can tell) is product market fit and finding your customer base. Once that's established, you have some momentum, leading to repeat customers and lower acquisition costs. IE: once an ad network has some existing customers to build a model on, it's cheaper to target ads on similar customers.
But that initial part is very hard. New privacy rules, like Apple's changes, are a good thing generally, but they make it more expensive for small businesses to acquire initial customers because ads are less effective, so you have to pay for more of them to find your customers. That gives Amazon (and other established competition) a massive advantage. They know everyone deeply and can target everywhere very precisely.
I've heard that the Shopify subreddits are well liked by merchants. Good info there.
There's also the Gurus that can provide some support for free, as well as you can hire an 'Expert' through Shopify to get even more help.
All this is to say the most groan-inducing phrase in business: you've got to spend more to make more. And there's no guarantee that you'll earn it back because business is hard.