Well, let me be honest with you, and this is going to be a little rant:
If renpy wouldn't make every single change of the UI so goddamn hard, the menu would look completely different, but every time I want to add a feature, or change something in the UI, it feels like I'll have to fight renpy. The way renpy's UI is setup is just awful, unflexible, even stupid in times. So many times I had to spend hours searching for undocumented stuff, even simple stuff that should be included by default.
If I had the time I'd create my own UI like I did with the main menu, but even that already took way too much time.
I'm not a coder, and I have updates to work on, so having to fight renpy every single time I want to change something, or add something, is really annoying and frustrating. That's one of the reasons why it took until last update until I changed the settings menu.
Btw I was actually looking into making a slider for only the dialogue, but I was already struggling with the color picker, so after a few attempts I just gave up. Probably will pick that up again with the next update.
I was thinking about removing line spacing already, but didn't because I didn't want to spend any more time with the UI, and yes, I know it's just one slider.
/rant
That said, take your time, no need to rush, I won't touch the game until I've finished the update for my other game.