6. Find out reasons why the custom workflow template was created
I guess that most of the custom workflow templates were created many many years ago. Probably those initial requirements are not valid any more and current requirements can be covered by the standard workflow template. In that case, use the standard workflow template or copy the standard workflow template and migrate the required changes. The point is keeping the same workflow container elements from the standard.
7. The last option is Fiori My Inbox
If your custom workflow template is completely different from the standard and there is no way to adjust, you can use Fiori My Inbox.