I'm thinking about replacing the SSD in my early 2015 MBP even though I haven't opened up a laptop since swapping out the RAM and hard drive in my old Core Duo MBP 10 years ago. The SSD in my 2.7GHZ, 8MB early 2015 MBP died, but not before I installed Mojave, and I've been booting 10.14 from an external USB3 SSD for the last 4 months (since early November 2018). So far, so good--I should be able to swap out the failed internal SSD for an NVME with a sintech adapter, boot up from the external drive again, and install the OS that way, right?
The problem is that I'm worried about my logic board. Whenever I add devices in addition to the external SSD to my USB chain, regardless of port or hub, my MBP slowly runs out of memory, freezes, and eventually restarts. It works fine if the external SSD is the only device on the chain. With even one more device plugged into the opposite MBP USB port, I can see the memory being used up in the Memory Clean 3 section of my menu bar, and each attempt to clean memory clears progressively less RAM until the mac freezes. This happens even if the Finder is the only program running.
I still need to try reinstalling my system software (if that will work without in internal SSD), but I might eventually have to get a new logic board. If I did get a new logic board, would I be able to install the NVME with the Mac OS already installed while it was on the old (mostly) working logic board? Or would I risk buying a new logic board having old firmware that needs my dead internal Apple SSD to upgrade its ROM's so they can read the NVME SSD, even if the drive is already formatted with Mojave running on it?