My DH has dementia. I remember him as a brilliant, responsible man and it is so hard to watch the decline. Any advice?

Home care sounds like a good option since you are not considering placement. A lot of home care quickly becomes more expensive than placement in most facilities so it would depend on how much care you would need to bring you relief.
Caregiver support groups have been helpful to some.
The truth is that you are dealing with what is the most common end of life scenario. Typically, husbands often become ill and require care sooner than wives. It doesn’t make it easier just because it is common.
I wish you well and relief from impatience which can be so hard to deal with.

My Significant Other of 10 years was well-known & respected in his field, too. He had been retired for 15 years. His slide into dementia & cancer was steep & swift - he died 2 months after his dr. suspected something might be wrong with his liver (official cancer diagnosis came a month before he died). He made his last informed decision 2 months before he died. After that, I couldn't help him understand what was happening, & I couldn't assuage his anguish. As much as I tried to be kind & gentle and explain things simply, it didn't always work. I felt guilty & angry. It wasn't until after he died that I fully accepted that his brain was damaged.
 
It’s important to get help. I had home health aides, including a live-in and eventually, an overnight, so I could sleep. Getting help didn't mean I was abandoning or failing him. I just could do no more. Getting help with his care meant I could be his companion - something as simple as sitting on a couch together after dinner without worrying about getting him ready for bed.
 
He became profoundly weak, & his mental state added a terrible layer. I wanted so much to reconnect with the man he was – I think that's a common desire. Was he the confused, angry, anguished man who couldn't remember how to operate his stair lift, who insisted on walking to the bathroom in the middle of the night even though he could barely walk, who accused my family of hating him because he wasn’t in a photo that was taken before we met, or who thought one of his adult kids was terminally ill? Or was the man I knew lurking someplace because he knew me, our relationship, our adult children & grandchildren? The truth was, the man I knew no longer existed because certain nerve connections in his brain were broken. I cared for him at home until his agitation forced a 911 call. He died 10 days later – 2/14/22. He was 83. It was a sad ending for a wonderful man.
 
My journey was short & operated at warp speed. But it also helped us avoid financial issues. You are at the beginning. No one knows what you & your husband will face, but I do know that honoring your vow to him and keeping him at home is not the same thing.  

Learn what you can about dementia. Be realistic about finances. Learn what services will be available to you and take advantage of them. Learn what coping mechanisms work for you. You’re not a saint: you’re human & you’ll make mistakes. Know your limits and try not to allow guilt and anger to guide your decisions. You might find Teepa Snow helpful: www.teepasnow.com.