I've recently been considering building a Shopify app(product renting) but I can't figure out if there is a demand. A similar app has <10 reviews. Do you have any advice?

Reddit has a good Shopify community - ask them if they’re familiar with the concept.

You can also do a 7 day trial of SEMRush and do some competitive research on the app’s domain.

See if they’re ranking for any keywords/how much search traffic the keywords have.

How have you stumbled upon this niche and recognized a need for this?

That was also my question