This blog post has been heavily leveraged but unfortunately is outdated. Please refer to the following Knowledge Base article for further guidance:
Updating Workflow and Forms for SharePoint On-Premises
In my case the service unavailable HTTP 503 error showed up on all sites except for the search, which is obviously on a separate web application. Which made me think maybe this was related to web application/application pool level since IIS reset did nothing. I opened the IIS Manager and checked the application pool to find most of it to be stopped. Everything started to work after i started the application pools manually.