A direct upgrade from R77.30 to R80.40 is indeed supported, both in migration or in-place CPUSE upgrade.
R80.x is more strict in making sure that data is correct, so some customers upgrading from R7x may encounter validations that need to be corrected. This is the same regardless of which R80.x version you upgrade to.
Note that in future versions another "stop" may be required to upgrade. This will allow us to focus on the newer versions and new content, instead of multiple legacy upgrade paths. When this happens, it will be recommended to upgrade to the latest supported version, then do the next hop to the final target version.