Anecdotally, the majority of people I've seen who self host are doing it to replace subscription services. This ranges anywhere from piracy to libre office. So, they're not gonna pay you a subscription for something they can do themselves.

The audience is niche because you're aiming at a subset of a subset of a subset of people. You're looking to sell this to someone who:

1. Doesn't want to pay for a service they can do by themself (self-hosters)
2. Has the knowledge and desire to handle networking (no amount of preconfiguration will make them not have to set up which ports their services need while allowing freedom)
3. But doesn't have the time/energy to do it themself
4. Can afford to shell out a rather large amount of money ($150 is a lot to many people, and as the other person brought up; you'll likely end up selling it for much more than this after manufacturing costs)
5. For a piece of equipment that is eclipsed by a 3 year old desktop computer from eBay

The amount of people who self host anything is already abysmally low - just look at the social media user count. There are more than twice as many people on r/pathofexile (which is already pretty niche) as on r/selfhosted. Obviously reddit isn't the end-all be-all of representation in that way, but you can definitely get an idea of trends from it.