My DSs' Grandpa died last week too, and we have the funeral on Friday. I wouldn't overthink things - you don't need to write too much, and I doubt that many people will read the cards.
How about, "Dear Grandad, I miss you very much and will treasure the memories I have of you. With love..."
My DS (8) wrote to his Grandpa the morning of his death. He wants the letter to go into the coffin / be cremated....but I think more likely that a couple of lines will be read out at the funeral. He said something like, "You were a great Grandpa and I am sorry you had to go. I hope you have a lovely time in heaven. xxx"
It is so sad, but FIL spent the last few months in hospital, a shaddow of his former self (all the marbles were there though), and was ready to go. A few years ago I thought he'd live to 100, but I am glad his suffering is over. x
My DGrandad died last week. My DM has given me a card to write to put on the family flowers. I am really struggling to think what to write. Do I write it to him? He won't see it. Do I write about my lovely memories of him? He changed in the last few years, and wasn't like the grandad of my childhood and early adult life. He was hard work and caused my little family a lot of heartache. I wonder if it would seem a bit fake to write about how funny he was, how wise and perceptive he was, how much I enjoyed his company, when that's not been the case for the last few years. But I miss him and can't believe he's gone, regardless of how he's been for the last few years.