Hi @arch,
Our visualizations plugin infrastructure has been going through a lot of churn since 7.5 -- I have posted a more detailed explanation in this Github issue, but the TLDR is that we recommend waiting to upgrade custom plugins until after 7.7.
In the meantime, you might be interested in checking out this thread from another user who is building a custom plugin in 7.6: Kibana 7.6: set uiState doesn't seems to work
Specifically they point to some example code they put up on Github which uses a custom request/response handler. (Worth noting that they are using Angular, not React -- I would strongly recommend sticking with React but hopefully that post is still informative).
Hope this helps a bit! We are expecting to have a much more useful (and documented!) API for custom visualizations coming soon in 7.x
Luke