Question: Need help starting a big project

I am planning to build a HTPC front end for our media library and retro & Steam games. I have the features locked down and have started writing the metadata scrapers.

But I am stuggling to decide whethe I should plan the UI/UX in full before writing code. Or write some of the bulkier subroutines first.

This isnt my first HTPC front end but its by far the biggest and most abitious and im stuggling on how to plan the project.