iCloud Photo Library I find is worth it for my needs. The phone will not have to keep the files locally, all originals go into the cloud storage side.
This is a good option. I've currently set it up like that. As I'm typing this, my phone is uploading all my original photos to iCloud and will replace the local ones with smaller ones. I can live with that. When I get my new phone, I will replace the "thumbnails" with the original photo's again. Thanks for pointing me into this direction.
There may be no windows native app, but it is pretty easy to log into
www.icloud.com and have a look at the photos. This is a very good option.
Also, thanks Matt for your input. Your option is a good one also, but I don't have a NAS currently, so I can't really use it. I do want to respond to this comment you've made though:
On a side note: Google Music All Access has a really annoying storage behavior. It'll store a cache of recently played songs, but that cache gets HUGE! My wife recently ran out of space on her iPhone again, but it hadn't been long since the last "purge." I found that Google Music was taking up around 2.4GB of space! I have no clue why they'd feel the need to cache that much stuff.
I found Apple Music does the same, which is why I quit using it at this moment. I'm currently on the three-month trial, but I don't think I will continue using it after that. The main reason is because it's an enormous drain on the battery. They phone becomes very hot even, so I'm guessing it makes the CPU run at its maximum level for this. This does not make it very good for daily use (in the office eg.). The second reason is indeed because Apple keeps local copies of the files on the phone for some reason. And having the little space left I currently have, this is not an option for me.
Also, thanks Tony for your guide! I will keep that in mind for when I have my new phone. I absolutely hate iTunes, and I try to avoid using it if I can. Which I why I like your way of making backups very much.
Thanks all!