You get two records in the context: context.record is the record being moved (dragged) and context.insertBefore is the record before which to insert it (it will be undefined, if the record is dragged to the end).
So having this information you need to calculate new assemblyOrder (or any other persistent field value) and set it. I don't know how your server process the data but the following pseudo-code idea should work:
context.record.insertBeforeId = context.insertBefore ? context.insertBefore.id : 'last';
// insertBeforeId would be a persistent field ignored by UI at client
This way the server would be responsible for reordering.
Other approaches are also possible, for example to calculate new assemblyOrder for moved records and for other records. Something as:
const newAssemblyOrder = // some calculation logic here
record.assemblyOrder = newAssemblyOrder;
I have re-tested it with the following changes to our Crud Manager demo and I think it does what you want. Let us please know if that solves your issue.
To add persistent field sortIndex we add it to resourceStore config: