Some thoughts:

Dementia is a catch-all term. It includes Alzheimer's Disease, vascular dementia, frontotemporal dementia and Lewy body disease. A competent neurologist should be able to do a differential diagnosis of these.

Eating in a healthy way is good for everybody, not just those with a form of dementia. It includes minimal processed food intake but lots of fresh fruit and vegetables, oily fish (good source of omega-3 fatty acids), whole-grain products (brown rice, whole-grain bread, etc), chicken (ideally free-range), red meat in moderation, pulses, nuts, eggs, etc.

Maintaining high-level cognitive activities like bridge is a good plan.

On a personal note, my partner and I got to care for her mother in the later stages of Alzheimer's Disease. My partner took out an Enduring Power of Attorney so she could make financial decisions for her mother when she became cognitively unable to do so. She also took out an Enduring Guardianship which meant that she could make medical decisions when her mother became incapable of doing that. For example, in the terminal stages her mother showed signs of breast cancer and the doctors wanted to intervene. My partner told them "No." which they had no choice but to go along with.

Finally, caring for somebody with any form of dementia is not easy. If anybody in the NT community is in that position, make sure that you have a good support network set up.