I think you are getting too specific and missing the big picture. Apple keeps blocking simple, low cost, solutions in order to make you pay more. The distinction between internal and external storage isn't that big - they both go against Apple overcharging for storage, and forcing you to depend on their services, such as iCloud.
Thanks for responding. Two main points:
1) You can connect a micro SD card to any iPhone via an adapter; all the ones that I looked up are all easily matching your (for modern standards) very slow transfer rate available on that card. The official adapters are $29, and have been around for quite some time. No conspiracy re: external storage. I believe the 16" pro and the studio also offer SD card slots again, a welcome addition for sure.
2) The very best NVME can match the internal storage on the Mac, from what I understand, but nothing else can. If they are moving the storage to a socketed solution, then it has to meet or exceed the current storage speeds. NVME uses M/.2/NGFF connectors. a quick search shows that they are much smaller than other solutions, but are still 2.3mm in height, 1.8mm of which must be above the MB, with additional clearance for the NVME storage module itself. The space reserved on the motherboard for the NAND chips is very small, so you'd need to work with the smallest NVME modules, if they are available at the comparable speeds.
Upgradeable internal storage isn't 'not doable', but it is extra challenges for a vanishingly small group of users. And that is before the changes/compromises that come on the software side.
To be clear, that SD card tops out at 220/mb a second, the internal storage of the MBP M3 is 6200/mb a second write and 4300/mb a second read... heck my welllll used 2019 Intel MBP (13/i5/16/1TB), 2/3rds full, still clocks at around 2000 write / 2600 read. So the latest MacBook Pro is clocking in 20x - 30x faster than your SD card, while my 5 year old storage is still doing 10x + the SD card's speed. Those things are not comparable. Most NVME on the market at a reasonable price is capped at around 3000/mb a second, which is still half the speed of the current MBP internal.
For external drives, there are much better, easy to connect and use options that are much better than the SD card - TB offers awesome performance, as does proper USB-C connected SSDs.
For RAM - In the past, I used to upgrade this on most of our Macs, at work and home. Then when it became unfeasible, I started buying the "next tier" ram for our work computers - 8 instead of 4, 16 instead of 8. The last Mac I did that for is my 2019 MBP. For the next round of computers I stuck with the base RAM config. We have multiple Apple Silicon Airs and Pros in addition to my intel MBP. I have one user/use case that is currently having RAM issues - My wife works from home, uses her M1 air for work, volunteering and home management - and she Does hit the upper memory limit on occasion; But she also runs multiple user accounts logged simultaneously, with each one having a pile of programs open (Excel, Firefox, Chrome, Music, Safari, Touch Draw, Metes and Bounds, Word, Mail, Messages, Pages and Numbers being the standard - with all the accounts having about the same open, except Metes and Bounds and Touch Draw - and don't ask how many tabs are open in each browser). About once a month she'll get a spat of "out of memory" messages, and she restarts... and they go away. In part, likely because she isn't shutting down or even restarting her computer. When she upgrades we'll get her the second step ram (she has her eyes on the 15 Air instead of the 14 pro...); when I upgrade I'll get the mid level Pro with its 'more base' ram and that will be that.
Techies see the high ram/storage prices and think it's a bait and switch but honestly I think most people look at it and shrug and just buy the base model... and most of them don't even wonder about a 'what if' for more storage and ram. Look at what's available on ebay etc for the post 2016 Macs, how few of them have higher than base storage and ram, if you wonder about what I'm saying.
Just because Apple doesn't sell the exact internal configuration that you desire doesn't mean that there aren't options out there, or that you specific use case makes sense for the consumer base a a whole. Thanks for reading.