NVMe is a nice solution, indeed. buying an old macbook with 1tb ssd as of mid 2020 is counterintuitive as it's resale value is still reasonably high. Instead, buying a completely new machine (windows) would make more sense, they're much more powerful and have dedicated graphics these days, with already 1tb hdd and 124 to 256 gb ssd for disk "C". in this case swapping the HDD to SSD makes much more sense (to me that is).
I do photo, video works, some illustrations and 3D design. Macs at this point become irrelevant for power users and professionals alike, especially those with conservative funds, and those who need every bit of processing power they could get per buck spent.
I will use external SSD with a usb 3, it gives me reasonable speeds, and I will attach it with velcro to the macbook plastic cover case. This should give me another year or two before a pressing need to chnage the machine and it's about 130-250 USD for 1 and 2 tb.
With regards to iOS being pre-installed, and how legal it is... I have no idea, they don't temper the iOS, so there shouldn't be any problems, also given how small the company is -no one would pay attention to them.
I hope someone would develop an affordable way to desolder and add more ROM and RAM to newer macs, past 2016 I believe, they are all onesie, nothing can be replaced. I guess at this point Apple can't compete with other brands for the above mentioned market nieche, they've decided to focus more on services rather than hardware...
Edit:
I still have ASUS ROG G750jx from 2013 with 36 gb RAM, 1tb HDD and 256 SSD and an old 770m Nvidia, the laptop still kicks ass in 3D renders and modeling, having base 2.4 ghz, I safely overklock to 3.4-3.8. Being so outdated it's bulcky and heavy, and this laptop outperforms my 2015 macbook pro mid spec, even the boot time is great, 5.6 seconds for a 7 year old beat-up machine isn't too bad. New asus and other gaming laptop competitors have put up a fight and dusted apple, I have mixed feelings about it. I guess switching to windows... would feel like a jump into 1900s, but the specs are too good for the price; and I could always switch to ubuntu or other distro to get a mac-feel like envoronment.
...That being said, Hatssd has outlived its relevance for my needs and my machine, unless I had the top spec with dedicated gpu.