Before anything else, I do use addons but I've tried this with both all the addons off and on. Also the world is 3.4 gb but when the update hit, the size of the backups jumped to 4gb. The backup which still works fine is 3.4gb. Ever since the update, my realms world has been broken. New chunks overwrote the old and there's holes in the world. I took a backup and that world seemed fine despite the fact it was also now in 1.17 but there's issues. The first day, I tried to replace the realm in the realm settings with the backup option. It took around 10 or so minutes but finally when it started extorting, Minecraft said that my internet was off. Seeing as, at the time, I was in a vc with my friends, I decided to leave it and close the other programs I had on. However it still didn't work. This was at night, when no one else was up. The next day it happened again. We began looking for solution and one of them was to replace the world with a different one then with the one you wanted. This is when my computer started crashing. Since then me and my friends have tried a lot of solutions but my computer/minecraft keep crashing no mater what I do. Please help.
(a month later) It's been some time and I would like you all to know that, actually, the problem is worse now. I closed the realm. And now every time I try to add the world, my entire computer always crashes. The realm is remaining closed. And I am very frustrated because I have heard no other solutions to this issue, all I have is what's on the internet and the very useful solution you gave me that has broken my computer every time I try it. I and my friends just want to play the world we made, on the realm we were on, the first world we all ever played on. But we can just do nothing.