This is less of a question but more of a reference in case anybody else experiences the same issue during their upgrade. We just completed an upgrade from 10.9.1 to 11.3 for our internal ArcGIS site which runs on seven separate Windows servers (1x portal, 3x server and 3x data stores).
We ran into a fairly fundamental issue when upgrading the portal element. The installer ran for about 40 minutes and then reported that it had successfully completed. However when trying to proceed with the next step we were unable to reach the portal website and it then became apparent that there was no Portal for ArcGIS windows service on the machine.
Repairing the installation (after another wait) didn't fix the issue so we did a complete uninstall and reinstall, which did fix it and enabled us to proceed with the upgrade.
The only resource we could find which referenced a missing Portal for ArcGIS service was this one in relation to the install folder name exceeding 34 characters (which it didn't in our case). We did attempt to run with the transforms file but couldn't get that to work, so just went with the uninstall and reinstall option.
Common problems and solutions—Portal for ArcGIS | Documentation for ArcGIS Enterprise
