It is possible that something in your backup is causing this crashing problem, or your backup is probably corrupted. I invite you to try uninstalling WhatsApp once again and reinstalling it without restoring your backup this time.
In case WhatsApp no longer crashes, restore your backup in order to retrieve your data, then you can export your chat history and media by email to keep a copy of them - see this page for the instructions. Once done, delete your backup from iCloud and reinstall WhatsApp.
Alternatively, because I understand that deleting your backup is the last thing you want to do, I suggest you try deleting some media, files and other attachments from your chats to see if that helps to avoid crashes. If that doesn't work, there's no other choice than to export your chats and delete your current backup unfortunately...