Only OWC make an after market SSD that fits apples connector and form factor. So it's that or buy a used one pulled from another Mac of the same vintage to keep the ultrafast speeds.
At the moment there are no upgrade paths that withstand much scrutiny. It might be better to simply accept the computer is non-upgradeable and instead look ahead to its replacement.
The problem with any non-Apple upgrade for a 2015 unit is loss of performance. The Apple units are faster (1.5 GB/sec) than third parties (0.4 GB/sec). And Apple uses a proprietary interface that other companies have to re-engineer.