How to update a MacOS installed in an external SSD beyond Sierra
Below are some possible solutions.

Replace the internal drive. This website shows replacement drives and links to installation videos.

Use another Mac with a working internal drive to upgrade the external drive to High Sierra or Mojave. Once completed, you Mac should boot from the upgraded external drive.

Try upgrading directly to Mojave, thus bypassing High Sierra.

Use an virtual machine to update to High Sierra. Here, the virtual machine would have a virtual internal and a physical external drive. Once the upgrade is complete, you should be able to boot from the upgrading external drive using the physical Mac.

Many answers explaining how to install Windows 10 on a external drive have been posted here at Ask Different. You could start with this answer. Since you have El Capitan installed on the external drive, you probably will have to adapt an existing answer.