Yikes. Scary to see this in the wild as one of my customers is moving to it and we'll likely be layering custom solutions on top of it...
Sorry to hear. Don't bill on a project basis, bill on a hourly basis or daily basis for custom solutions, and it might make sense to bring on outside expertise because there are quite a few footguns. Either way there will be a learning experience.
People make a lot of money doing that
Honestly in this case I'd rather make less, and not work with servicenow :/