I haven't been able to find the solution to my issue anywhere, so here goes:
I have a 15" MacBook Pro (mid 2015) and it's at all points a great device, with the only drawback being low storage (I only have the 256GB installed).
I am aware of OWC's solution and I almost bought a 2TB SSD, but then I remembered that the new macOS runs a bit differently, if I understand this right. I am not sure if the OWC's SSD would be even compatible with Big Sur (I also read somewhere that it's also completely incompatible with Windows 10 already, so no workarounds there?).
So, does anyone know how things stand right now with upgrading storage in "older" MacBook Pros in 2021?