What customer acquisition channel should I focus on?

Social media for sure! I'm thinking of professional and hobby Facebook groups. Also make sure to spread samples of the templates available so people can see how pretty they are and short videos of how easy they are to make.

It seems to me that your target audience is event planners who help people organize surprises for their loved ones. They usually ask for specialized notes and stuff. So maybe try scouting for forums where they hang out on WhatsApp. Or you can try a partnership with online cake vendors.

I know you asked for advice on customer acquisition channels. But here are some extra things. And please, I mean everything I'm about to say in the kindest way possible.

1. I think that you may need to first confirm that you have a substantial target audience. From my experience as a demand generation marketer, it's way more difficult to generate demand for a niche than it is to generate demand for a product. Your product seems very unique and you may have to go the long route of generating demand for the niche. So maybe you should try gauging interest first, before you try acquiring customers. That way, you know what works early and you can focus your efforts.

2. Letter writing isn't particularly a popular thing these days. You can do a sentiment analysis to check if I'm wrong. The fact that it is AI-powered isn't enough to generate interest. So if you spend time defining the problem using a problem formulation method, you may discover another angle to it.

I think for you SEO and direct mail could work, All the best!