MacBook compatible SSD conversion

I was under the impression the OP understood they need an adapter because that board is too old for native PCIe SSD support.

The two blue slots are marked PCIe x4 and the two smaller slots in-between are marked PCIe x1. I would use one of the PCIe x1 slots for the card Mother bd image. Only the older SSD's PCIe/AHCI not newer PCIe/NVMe drives is that what you're thinking?

The other option would be a mSATA adapter but I don't know of any that supports the Apple SSD's.

You need chipset support for NVMe so it's only going to take AHCI PCIe without a board and CPU upgrade. I don't think you can make NVMe work on old platforms.

Correct! If you review this guide The Ultimate Guide to Apple’s Proprietary SSDs You'll see the Early 2015 13" MacBook Pro Retina system comes with a PCIe/AHCI SSD

My fear is the performance more so then compatibility. The OP knows it's pre-NVMe so we can tell him about not being able to use one of those drives; that's why I warned them that the adapters tend to slow the SSD down.

I think your solution is better, but I went with the adapter and warning because the OP seems dead set on it so my priority is to warn them. I suspect the same issue will appear with the interposer you linked.