Apple are almost certainly never going to offer upgrades to the Studio. They want people paying $300-400 per TB when they buy the machine! So I think upgrading at any price is highly unlikely, unfortunately.
Apple also very clearly wants all their machines to be sold-on into the second-hand market, and new ones bought afresh – that’s where the money is for them as a business. This kind of works for users at the moment, as used devices sell very well on various used marketplaces like Ebay et al. And Apple prove this is their strategy for user upgrades, as they even offer their own trade-in system – which obviously offers very low trade-in values compared to selling yourself, at the benefit of convenience to those who can’t be bothered or don’t want to sell themselves.
re. music storage.
I use an SSD hanging off my Mac, as you can now ignore SATA SSDs and go straight for NVMe SSDs at 4TB (~$250; commonly found) or 8TB (~$500; best price) inside an external Thunderbolt 3 enclosure. Make sure the enclosure you get is a Thunderbolt 3 one (~$125) rather than vanilla USB-C one, as they offer vastly different performance benefits.
Obviously for storage, the NVMe SSD you’re after doesn’t have to be a cutting-edge 7GB/s model (with a v.high price to match), the cheaper slower ~3-4GB/s ones are the ones to buy.
I like this, as it makes adding new content very quick and easy, and makes for a super-smooth user experience in accessing/playing content, without the bottlenecks of slow HDDs. Backing-up to spinning rust HDDs is still cheapest for double-digit (10TB plus) needs.
Just keep in mind that even if you get a high-quality TB3/4 enclosure, perhaps even with active cooling, that TB bus is never going to push more than 2800 MB/s (and more like 2100 MB/s on write) to or from your external SSD. Howard Oakley has some examples of that here.
Just keep that in mind if you find yourself wondering if you should pay extra for flash that’s rated to do 7 GB/s vs. 5 GB/s or just 3.5 GB/s. There’s substantial differences in those prices:
I know that Hoakley article; it pretty much explains why TB3 enclosures are preferable to USB-C ones.
Of course directly-on-board with Apple can go far beyond the bus speed maximums of TBolt, but you don’t need that for everyday media storage needs – hence Apple charging those $3-400 per TB prices, we shy away from if possible.
And yes, you don’t those 7GB/s NVMe’s for media storage. Especially 4TB and 8TB sizes.
I’ve never had a problem with using spinning tin for music and movies. I find no noticeable delay in playing them. Photos are a different case. If I haven’t accessed Apple Photos for a while so that there is nothing in a cache, there may be a delay of a minute or so. However, once the app has started, all operations proceed quickly.
So, don’t fret aboiut speed of your storage for data that is accessed passively. Data that you manipulate may benefit by being stored on faster storage.
Paradoxically, I do see the speed penalty in using HDDs for backup, but creating or modifying backups quickly is not usually a priority.
Doing more intense work with photos, I realized the delay with having the Apple Photo Library on spinning tin was becoming intolerable. So, I have moved the library back to the MacStudio SSD. The response is now almost instantaneous. So, I recommend NOT putting a photo library on an external HDD. It’s probably fine on a fast external SSD.