Well on here would be my answer. I can't find a link to the latest survey done on here but I seem to recall that a very significant number of members on here are caring for a parent or grandparent I'm not even sure that caring for a partner are the majority on this forum, look at the number of posts who say mother/father/parent in law.
The issues you mention like eating are pretty universal, how does the relationship matter in that context.
Just so you know I'm on here currently as my wife has AZ but for the last 3 years of her life my mother lived with me and she had an unspecified type of dementia so I've been where you are as a child with a parent with late stage dementia now I'm a partner and the difference is...?
Many on here have done the parent and moved onto the partner sometimes more than once, the issues, caring, support and compassion are more important than the relationship in my opinion for what that's worth.
If as you say someone isn't eating then is the advise from a partner less valuable than that of anyone else, why would you feel out of place taking advise from anyone irrespective of their relationship to the PWD.
I think the site you're looking for that is "specifically at the offspring of dementia sufferers" doesn't exist because caring is universal, it know no barriers of relationship, age, colour, religion or anything else any issues anyone has with thins like that just cease to matter when you're faced with AZ.
I hope some of the people in the same position as you will post, but as I say the number I believe the number of people on here who care for a parent or parent in law are the majority.
K