If the laptop is already running macOS 10.13+ (hopefully macOS 12.x Monterey), then you could replace the original Apple SSD with a third party SSD upgrade from OWC. The third party NVMe SSD requires the laptop to have a system firmware update which is only available from macOS 10.13+ and macOS 10.13+ must be used with a third party SSD. If macOS 12.x Monterey is not already installed, then you should upgrade to macOS 12.x before installing the third party SSD since the Monterey installer will refuse to install until after the laptop's system firmware has been updated by the Monterey installer.
https://eshop.macsales.com/shop/ssd/owc
Keep in mind that macOS 12.x Monterey is the last version of macOS that Apple will support on the 2015 Mac, plus Monterey will only receive limited security updates for the next two years. Of course the laptop can still be used beyond that time, but some apps may no longer offer newer updates since many developer's only support the versions of macOS that Apple fully supports (usually the current OS and the two previous versions).
Another consideration on whether an SSD upgrade is worth it, is whether your laptop has 4GB or 8GB of memory. 4GB is not very much memory. Using an external SSD may be a better option since it could be used with any other computer as a data drive after the 2015 laptop's usefulness ends unlike the OWC SSD (I'm not aware of any adapters or enclosures for using an OWC NVMe SSD externally -- OWC's own enclosures specifically warn that the OWC NVMe SSDs are not compatible with the OWC enclosures).