What is your current phone's storage? I ask because while many Android phones do, many do not have memory card slots and are fixed as well (I'm assuming Android since you say this will be your first iPhone). Also keep in mind if you ARE using Google right now, you can continue to use things like Photos to hold all your photos and keep the phone clear. I have 19GB of free storage with Google due to years of having Android and getting extra space for doing the account security checkups and stuff so I continue to use that for email and full res photo backups (counts against storage). My iCloud backups for the rest of the stuff on the phone is then only about 1.5GB of the 5GB of free space.
It sounds like your biggest issue is a lot of music. I faced the same dilemma when I went from my 128GB 7+ to a 64GB X but I was only using about 40GB of the 128 after a year of use. About 1/3rd of that was music and I realized I wasn't listening to a lot of it so I just didn't put it back on the X for right now. So you could always go with the 64GB 8 and do a little extra management of your music and not keep stuff you don't listen to very often on the phone.
You can actually do the same with apps you haven't used in a while. It will "uninstall" the app itself until you want to use it but saves your data/settings for it. I used that to get rid of things I never use like Garage Band that take up over a gig of space.
Now that's an awesome insight right there. About 40 gigs after a year. I am currently using a Xiaomi Mi Note 2 - 6GB RAM and 128 gigs of space. As of now, it reads Available: 68.81gb out of total 128. While I can still take care of the music, my question to you would be apps. If we were to just focus on this for a bit, do you think...say 10 gigs would be enough? I don't play a lot of games, but yes, given the option, I would want to play one on the iPhone since it honestly looks just awesome on it.
My photos are all backed on Mega so I am not very worried on that. However, I do realize one thing now. That the storage is going to fill up as we go on. It's not going to be constant throughout when it comes to things like apps and photos.
Also, one more thing. I would be using this for about 2 years at least.