When will I know if I develop dementia? I want a place to talk about my future care needs.

The first thing you have to put in your head is that dementia is not a normal part of aging. Dementia is a disease that some times affects old people, but not everybody. You're young and have a few more decades of life ahead. Think positive, don't become a hypochondriac. Find activities and hobbies that can keep you busy and entertained with happy thoughts. Get involved with friends in social activities. Don't forget to eat a healthy diet and exercise every day.

Your best bet is to develop a solid relationship with your MD and to see that MD twice a year. Just let him or her honestly know that due to your history you have these fears and feelings. At 65 you are very young, and you have decades to have a quality life that should not be needlessly be taken up with these unrealistic fears. Meanwhile form a group of friends you care about with a common interest whether political, reading, chat group, cooking, knitting, sewing. With this group develop mutual support and chat group. Tell them your fears; listen to theirs and begin with extreme honesty.
You have no family? That means you make your own now. A great resource in my own city is our library. Check out senior centers. Check out places to volunteer when the plague is over here, such as animal shelters, such as nursing homes. Keep busy and active.
Having been through cancer and a survivor of 34 years I well remember when EVERYTHING was cancer, whether forgetting my keys, misstepping, saying a word wrong, forgetting something, having an odd pain in my shin. That may be the case for you for a while. But the answer is usually getting out.
So see your doctor. Often now they are doing video chats. Discuss your feelings with him or her and ask to see him or her, or to have a video chat twice a year, and tell them you would like testing periodically. Then get out there and make a support group whether through a church or a book club.
I wish you the best. I am so glad that your time of caregiving brought such love and satisfaction.

May I suggest that you have a frank discussion with your physician and ask him or her to tell you when s/he feels you may need assistance? If possible, see a geriatrician and explain your concern. That doctor can arrange for you to see a neurologist to study your cognitive functioning and give a diagnosis. You probably are a long way off from any dementia but one never knows. Also, it is hard during the pandemic but when things loosen up you should join your local senior citizen center or group which can offer activities, exercise classes, trips, etc. Search online for groups that you might join virtually.