Although this isn't the right topic for your phone model (this is the 'regular' S4's topic, the i9500 / i9505 / i9505g), I'll answer your questions because there isn't a S4 mini topic on this forum (or at least I thought so

).
However, i was wondering:
- I'd like a clean phone with little to no bloatware. Are there any applications and functions i need to remove or disable?
You can basically turn off / disable almost any function you don't feel like using. To remove the apps you have to delete the corresponding .apk files that are placed in the /system/app folder. But.... you need to root your phone to be able to do so.
- What's the best way to extend battery life? I read about a black background, and apps like Greenify or Juice defender? Any options i need to turn off manually?
Juice Defender is bloat itself. Greenify on the other hand is a remarkable piece of software that, if you can live with longer loading times (because the greenified apps are closed and not running in the background anymore) does extend battery life. I believe that recent Greenify versions also support non rooted phones. Please note that for greenifying system apps you need a rooted phone
and the donate version of Greenify, as wel as the Xposed framework.
A dark background is indeed better for your batterylife since the S4 mini has an amoled screen. More darkness means less effort to show light = less battery usage.
- Any other important things that'll improve the user experience, like must-have apps ...?
This all depends on your needs. I myself cannot imagine not having a rooted phone with the Xposed framework and modules, because the ease of it completely renderes the use of heavily modified custom roms and mods useless.
So basically, here's the deal: i don't know sh*t about android, i'd like a clean phone and in general i don't really care for many of the functions of a smartphone. It will serve as a glorified dumbphone and mp3-player. How do i make the most of my phone and the juice?
If you really like a clean phone, I'd suggest you read some more about Android, rooting phones and custom ROMs. There's several 'strings' of ROM building, based on Samsungs firmware, CyanogenMod, AOSP and AOKP. The last three are usually the most clean ones. A good place to start would be XDA:
http://forum.xda-developers.com/galaxy-s4-miniThx guys

Greeting and thanking is not necessary here on GoT

BTW: I just moved here and you guys seem pretty savvy so that's how i ended up here. Please don't make an effort and just respond in dutch. As long as you're a little gentle with your vocabulary i can understand dutch just fine. For now i'm more comfortable typing in English.
Ow shoot, already answered in English. Wasn't a real effort, so no big deal.
Good luck