Unfortunately yes I do have a hard requirement for 7.8. This is being developed for a government contract and they require 7.8. I've looked at 7.10 and things do appear to work better for plugin development with 7.10 but I have to get it working with 7.8.
I'm also getting these errors when starting Kibana after developing my plugin. If I remove the dependencies object from the package.json file, then my plugin loads and runs on my development copy of Kibana 7.8. I guess my question is do I have to do an npm install of the other node modules? It appears I do not.