Volto for editing only?

Is it feasible to have editors use the Volto interface/ui to manage content on an older Plone site which exposes the Rest API?
What are the gotchas or concerns? I know that Volto's block editor stores things in a json-style format but I think there's a way to keep it as rich text.

Thoughts?
The reason for this is to keep the current site and theme alive while we move over and give the editors the benefit of the Volto editing experience.

I guess you will have to convert the Slate/DraftJS blocks definitions in JSON to HTML. There are libraries for that which you can use on the frontend. You will also have to render the full blocks layout (e.g. when you use a grid block solution).

I would suggest looking at the Gatsby Plugin.

Another option would be to "just" implement the old theme as a Volto theme.

Yup...

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