My mom left strict no funeral instructions. She told me verbally the immediate family could gather and have some sort of service we designed. Her most important paper that she read was a mediocre poem that touched her soul- it was poorly written but expressed how religion had held the person captive at one point in life, how the person was rejecting reliigon and exultant. Altho it was so bad, it had impact and I cried. She demanded that it be read as her statement when she died.
I am Episcopal. She gave me permission to read some famous Psalms and recite Amazing Grace under the guise that it was my way of coping, not hers.
My brother in law, NOT a child, NOT liked by her, is an ardent RC and demanded a Catholic funeral. She would attend E. services with me. Once you received communion in all her church going with me. I was not present. It was at an AIDS service in San Francisco. My brother and sister clearly knew her long time wishes. I showed them her handwritten will. No one wanted to respect her feelings, besides me. My brother in law is so male alpha. I could not believe they would not honor her wishes. So I stopped being nice. Announced I had standing to seek a court order stopping a RC funeral. He had zero votes. It was tense. They were cowered.
My recommendation is to write detailed instructions for your wishes and if you have bypass traditional family, do so. Her whole life was a rejection of religion. Also, she was raised a Witness. Being Catholic was far worse than being Protestant. She was expelled from school and present at the riot in MSG stirred up by the RC. Over my dead body.