Seriously, the question provides way too little information for a reasonable answer. 80 TiB is meaningless - the number of documents, complexity of the file/folder structure and requirements for any kind of metadata mapping/extraction is way more important than just the storage space currently occupied.
The remarks by are correct so far. You want to avoid creating too many small transactions to improve performance for any sort of migration. But you also don't want to create transactions that are too large because indexing speed and system requirements (memory) are affected by this. Generally you should aim to have a 2 or low 3-digit number of documents in one transaction.
The bulk importer is only relevant if all your content and its metadata has already been exported to a file system in a specific layout / format (concerning the metadata files). If your existing content is stored in Documentum it would not help at all unless you already have a process to completely export all the content and metadata onto a file server / system.
Any migration of a large-ish amount of existing content should ideally be implemented / supported by professionals that know both platforms (or complementary professionals for each platform).
