Customize and Preview

When the aspect ratio of the background is not 16:9, it crops it. Letter box (with the background color) would probably be a better default.
This is now doing a “cover” background which expands the whole canvas (16:9) centered and crops the borders if necessary. I think this is better than letterboxing, but I could add a feature to choose this.

There should probably be some way to add a title, or a title slide image in addition to the background
This is on the roadmap! (tracked here)

You really shouldn't advertise it as customizable when all you can customize is colors and background. It's ok to have an opinionated tool that doesn't offer a lot of customization, or still have the features in progress, but please don't call it fully customizable until you can at least set things like font face, outline color, lines per page (including ideally customizing page breaks), etc.
A lot of customization options have been added: fonts, colors, outline width, letter spacing. Lines per page is tricky because I need a default that works well for all kinds of lyrics (short and long lines). So for now this is what we have.

You may actually be going too high with the quality settings on the video. It's fine for uploading to YouTube, but people will potentially recompress it to distribute it offline. 320kbps is overkill for AAC, 256 would be more than enough, and converting 44.1KHz audio to 48 is just wasting space. I think there's also room to optimize the video output, but I don't know as much about the specifics there.
I think it’s better to have a higher quality video. People can always compress it if needed or even for YouTube upload is better to upload good quality videos. A standard video with a colored or still image background is less than 50 MB. I think that’s average for today’s standards.