I have read here that I cannot restore a backup on a different major version
Although it’s not officially supported, most times, features are added to newer major versions instead of removed. This means that you can mongodump from an older version, but doing mongorestore might not be as straightforward, but still within the realm of possibility.
Having said that, if I’m in your shoes, I would do a complete mongodump on each major version step, so that your dumps are always up to date, and ready to be restored just in case the latest upgrade step doesn’t work out.
Hope this helps!
Best regards
Kevin