Our company admittedly wasn't an ✨ indie hack ✨, but we made some easy mistakes that are broadly applicable:
1️⃣ "UI/polish" is not sufficient differentiation for your product. Your product should have some killer feature that will make it a perfect fit for a group of people out there - a nice looking UI is almost certainly not that feature. However, it's still worth putting in the time to make your product look nice: it gives your growth a tailwind because people will want to try your product. How do you make something nice and ship it fast? Keep in mind, that...
2️⃣ Potential customers extrapolate from the quality of what they can see. The majority of customers that evaluated our product made a purchase decision based on a tiny sliver of the total surface area. We could have launched and iterated much faster had we focused on answering "What needs to be built to sell to customers? How can we avoid building the rest for now?". As an example, a simple "Send an email to support@example.com for any account management help." can help save you weeks of building account management screens.
(I wrote a blog post a few weeks ago exploring this idea in more detail.)
3️⃣ Hunting for whales will kill your company. This is doubly true if you lack existing relationships that would lubricate these sales. Large companies make purchasing decisions that span months, taking up valuable time that you could have been iterating in response to customer feedback. If a large company wants to adopt your product, that's awesome: just don't bet your entire company on landing that sale.
Happy building!