Been there before with that data volume on the business side. So the first is your hosting and technical implementation for performance due to the indexing on save or reindex if direct database import. The second is the management and approval flow for product management.
The performance you need high quality architecture and preferably separate admin servers to offload from the web servers. You can use EE which solves quite a lot of this but you can also do the same with CE, you just really need to know what you are doing.
On the product manager point there are a few options, the simplest is a direct Excel to Magento import where data entry control a specific set of products. The second is a passthrough that take files, runs them through mapping and loads to Magento. The third and most complex is an external PIM, however that becomes more complex as you need to config the PIM for the products, batch the updates with only changes and then have sync connectors to Magento. We use all three types depending on the scenario having used the first to 70,000-100,000 products (you need 64bit Excel) and the latter two to 300,000-500,000.
You obviously have Magmi but that's a technical tool and you still have the reindex problem as it's better at data load which is more technical than data maintenance which is business focused. The main thing is that your situation is a business process problem, the technical can always be mitigated but it sounds like you need an architect to understand your process and tell you what to do. The ones we use via via take around 2-5days for an overview and core issue identification but you really don't want to know their chargeouts. It all comes down to how the pieces are added together for you budget, technical installation and business processes.