Mnemba was not AT ALL rustic in our visit last year. I did not use my sandals even once during our stay as grounds were very well maintained. It probably had a major refurbishment recently. Diving was very convinient, luxurious and private.
As long as wanting to stay closer to Stone town, Serena is probably the best option as you can walk all around town very easily. Other luxury resorts will require a bit of drive. You may want to look at Baraza and The Residence. Best would be to combine Serena with a more secluded resort.