Well, I guess the way I understand it, ‘everything’ includes pro features, such as row limits, sheet edit limits, storage limits, larger usage limits on some components, access to some of the pro only components, etc. I don’t consider the privacy level to be part of that as it’s implied by the name of the plan.
Is there a specific Private Pro features that you are looking to use? The biggest advantages for a Private Pro plan are complete data security with access only to those whitelisted users, along with the use of Role based security. Also keep in mind that a Private Pro plan charges by the active user count, so if it was opened up to allow absolutely anybody to sign in, you could be looking at some high per user costs.
I think you may have to explain tiered access a little more. A private pro plan may not be what you are looking for, but if we understand what you are trying to achieve, it may help to get you the proper solution. One additional question is, how secure does your data need to be? Is this something that could be achieved with some simple filtering and visibility, or is this vital data that should be prevented from downloading to a user’s device in the first place if they should not have access?