The only way I can imagine fitting a mixed model, rather than a fixed effects model, that includes the predictors you have is if the census regions included are not exhaustive of all possible census regions to which you wish your inferences to apply (G-side effect), or that some sort of spatial correlation is to be applied to that variable (R side effect). In either case, a generalized estimating equation (GEE) where census_region is the clustering variable may be more tenable in terms of model convergence and run times.
SteveDenham

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