How do I move off WordPress without losing rankings?

Rankings survive a migration when every old URL redirects permanently to its closest equivalent. Content moves easily and plugins do not move at all, so the work is deciding which plugin behavior was actually load-bearing and mapping every URL before launch rather than after.

The longer version

A complete redirect map is the single thing that separates a migration that keeps its traffic from one that does not. Every indexed URL needs a permanent redirect to the page that now serves that purpose, including old category pages, paginated archives, attachment pages and anything with query parameters.

Plugins are where the real decisions are. Most sites carry a dozen, of which two or three do something the business relies on and the rest are historical. Each one needs a decision: rebuild, replace, or drop deliberately.

Expect a temporary ranking wobble even when it goes well, because search engines need to recrawl and re-associate the new URLs. What matters is whether it recovers within a few weeks, and that is determined by the redirects.

What to do

Export the full list of indexed URLs from your analytics and from search console, including every page below your menu.
Map every one to its destination before launch. Anything unmapped is a decision you are making by accident.
Audit the plugins and decide explicitly what happens to each.
Keep the old site reachable until the redirects are verified working from outside.
Watch coverage and rankings weekly for the first two months, and fix any redirect that turns out to be wrong.