The final implementation, that will resemble the one in the post I linked above, will be different from the old tutorial.
In particular
R = dolfinx.fem.FunctionSpace(domain, ("Lagrange", 2))
is not an equivalent of the former
R = FunctionSpace(mesh, "R", 0)
(i.e., a FE space with a single global degree of freedom). The former "R"
space has not been reimplemented yet in dolfinx
, and that is one of the reasons why the new implementation will need to be different.