I've found the #1 trick to finishing a side project is to build it out in the open by writing a blog (or podcast) about it and building up an audience who are interested in seeing you finish it as you go along. It also gives you an in built audience to launch with who will use it - rather than launching to a loud thud of silence - which sucks and feels like the long dark night of the soul.
Then you’ll just have a bunch of indie hackers following you. Not the actual customers? Real customers only care about your journey in hindsight. People want to know how Apple etc started, not many will care for journey of a new tech until it’s released.
I've always found that early adopters and innovators are very interested in being first customers and following your journey.
It also depends on what side project you pick. So, there is some nuance for sure.
[edit removed link]