The real answer? Buy a phone older than the 15. You'll never get on-device intelligence. My 13 Mini takes great pictures, performs perfectly, has great battery life as a non-gamer. And it should get at least 3 more years of iOS updates (maybe more? phone specs have really leveled off).
Anything newer, Apple and Google will start pushing intelligence on you simply because your phone can do it.
You could try GrapheneOS as well on a recent Pixel.
I use Jellyfin to host my music, and Finamp on my phone to browse and listen to it. Finamp supports downloads as well, so you can listen to your music offline and away from home. Pair that with a self-hosted VPN to access Jellyfin away from home and you've got most of your needs covered!
GrapheneOS kills support when Google kills security updates, I believe. Source: my Pixel 4a came out in 2020, and Graphene already strongly recommends against using it and dropped updates entirely a few months ago.
Lineage and Pixel Experience ROMs are better at long-term support. But any custom ROM on older non-officially-supported phones is vulnerable to firmware exploits, since those fixes are typically distributed as binaries by the hardware manufacturer (Qualcomm etc). So I understand why Graphene drops support so quick, since they want all Graphene users to benefit from strong security practices.
Longevity is worse than non-folding phones, simply because the hinge and folding screen are wear parts.
Camera is worse, because there's less z-distance for lens depth. If they push it with a bigger camera bump, you wind up with a maximum thickness of easily over 15mm very quickly, even if the rest of the phone is thin, that hurts.
Battery life is worse, because two separate batteries with puncture protection, folding screen, digitizer, etc in between are heavier and less power-dense than a single large battery.
And the price is higher because there's more screen, more batteries, more materials, and more R&D to pay back in the hinge and fold mech.
The Z flip is the most tempting to me simply because the height and width are far more attractive than current monster-sized phones. But it's disingenuous to pretend the plastic inner screen (yes, that's literally what that top layer is, not glass), the cost, the inferior battery, and the inferior camera aren't serious compromises.
And all for what? So I can have a giant inner screen? I think my (smaller than any modern phone) phone is already too large, too addicting, and too tough to use with one hand. For serious tasks and videos I use my laptop.
I believe it is awesome for you. But there are downsides.
Of course, nothing wrong with it. In fact it makes OP's quandary a lot easier! I'm looking into something with 20TB or so of capacity myself, and that's given me an appreciation for how much simpler it is to solve this problem at 2TB.
2TB is insanely small for a NAS. At that point, you could honestly just run a Pi 5 with M.2 HAT and a 2TB SSD for something like $200 total. Could always buy a second Pi for mirroring and even locate it in a friend or family member's house for mirroring and backup.
I use a Pi 4 with 7 TB of external SSDs just fine at home. It also hosts a pi.hole ad blocking server, my 1TBish jellyfin music streaming collection, my network share for kodi, an always-on VPN for my phone and laptops, and a few other small services. I'm sure I could upgrade for better read/write speeds. But everything is performant enough as is, and it's completely silent and fan-free in my living room by the router. Honestly for most services a Pi with a passive cooler will perform admirably.
Syncthing on my home server, synced with each device I use for notetaking, has been glorious so far. I wish Obsidian would offer Sync for a cheaper rate, because I'd pay if it felt like anything near the cost of actual sync and storage. But Obsidian's cheapest tier is more expensive than my email hosting!
Just be sure that the second server in the list is also a black hole. If you don't, all black holed requests will fallback to the second DNS... which, if it doesn't also black hole them, will wind up serving you ads and defeating the point!
Personally I find a single Pi is just fine for DNS. It only takes like 10 seconds to reboot. Less, if you use M.2 storage via a HAT or boot from USB! That's pretty fine downtime. But if you're afraid you'll knock over the network and get yelled at by your family or housemates, best to use a backup :)