Slow down -- very little has to be decided the first month. Get the specific diagnosis type, since Alzheimer's, vascular dementia, Lewy body and frontotemporal dementia progress differently, and put power of attorney and medical power of attorney paperwork in place now, while capacity is clear.
How families here handle this
The legal paperwork is the part families most often put off, and it is the part with a real deadline attached: once capacity is genuinely in question, financial and medical power of attorney become far harder, sometimes impossible, to execute cleanly. Doing this in the calm weeks after a diagnosis, rather than during a later crisis, is one of the few truly time-sensitive steps in this whole process. Everything about care levels, memory care requirements, and long-term planning can generally wait longer than the anxiety of a fresh diagnosis makes it feel like it can.