That’s a common problem. My mother and step-father kept everything. My step-brother and I had an estate sale, but still only grossed a couple grand. Better than nothing, but it was a lot of work.
I went through the same thing a couple of years ago. My feeling was: if it gives them joy, who am I to argue? When the time came to downsize (dad had passed, mom moved into a care home), I had a friend (odd jobber) come in to clean, sell what he could and toss the rest. I paid him his flat fee and ended up with a couple of thousand bucks. He also detailed and sold Mom’s car privately for a commission. Well worth it.