Linux

The solution for the music I've arrived at is based on OpenSubsonic.

I'm running gonic (Navidrome works, too, and there are other servers such as the original Subsonic) on my server. I'm using Android, but there's probably a similar iOS app, and the OSS Tempo app. Tempo can stream, but it'll also let you download and cache music locally - including entire playlists. So I have a "Phone" playlist that I add music I want to sync to my phone (my music collection is 84GiB, so I sync only part of it), and occasionally download it. Tempo is smart enough to only download songs it doesn't already have.

This is only 1-way. There's no facility to upload music to the server using OpenSubsonic. Also, caching only downloads; if I remove a song from the playlist, it stays on the phone. It's not a true "sync".

I used to use SyncThing, but maintaining include/exclude lists was far more work.

For SMS, I wrote a program. But it requires two different OSS apps on the phone, and I doubt they exist on iOS, they don't sync messages received when the phone wasn't connected, and while one runs reliably, I have to keep restarting the second (it doesn't persist well). So it's not really a good solution, for many reasons, but I can't bring myself to do mobile development anymore so unless an alternative that does what both SMS to URL Forwarder and RestSMS do all in one app, and include caching unsynced messages, it's not likely to improve. It's enough that I can respond to SMSes from my computer, which is the big thing.

KDE Connect works great to sync SMS from Android to PC, and they have an iPhone app, although I never tested it.

I don't have an iPhone but my daughter does.

For music I don't sync anything to her phone. I run a navidrome server and set up an account so she can stream whatever she wants whenever. I think she uses isub as her streaming app. It does allow you to download and cache files from the server to play if you are offline

As for sms, I don't know of any way to sync in Linux, but if you use Windows, the phone link app works, as several of my coworkers have set it up. I know it installs some piece of software on the iPhone you want to sync to, maybe you could do that and try running the phone link program with wine?

I also know kde connect has a link for iOS. It's not perfect but it will do the sms linking thing.

1. I run a Jellyfin server and use the FinAmp app to stream music. I know FinAmp will let you store music for offline playback.

With modern iPhones? I haven’t found a way that works unfortunately. Apple wants to keep their users in the apple ecosystem, so this is always going to be an uphill battle trying to work with Linux.

For music I know rhythmbox use to have good apple connectivity, have not seen it work for any of my current devices sadly.

There is a program called blue bubbles that should bring some iMessage functionality over. I have not tried it personally.

https://bluebubbles.app/

Sadly the best thing in my experience is to keep a mac or pc that has iTunes available for most things. Your mileage may vary though and you end up having better results. Good luck!

IMO There are a few (not so great) options.

Cider is a decent alternative, especially for local files.

You can also use Apple’s web player for anything there.

Some other (even worse) options are windows VM, or WINE.

I'm sorry I can't help you.

But If you like Linux, sell your iPhone, buy a Pixel and install Graphene OS. It's the closest thing to a Linux phone that is actually secure, private, FOSS and daily driver worthy.

I don't have advice for an iOS user. I will say that Android is far more flexible for Linux users. So, maybe go Android?