To build off the above answers.
You have the ability to onboard your clients in bulk and that's exactly what Microsoft create the Azure AD B2B (Busines to Business) API for. You can upload a list of addresses and those users will get an email inviting them to your tenant. This is a little different from the external sharing in OneDrive/SharePoint, but essentially works the same. Once they have validated their accounts you can assign them permission like you would an internal user.
One of the benefits of using Azure AD to onboard external users over the native Sharing in SharePoint is that that sign-up experience is better. In the Background, if users don't have an azure AD instance for a domain, Microsoft create one transparently and creates their account there. It works a bit better than going thru the process of creating a Microsoft account. Additionally, new rollouts allow you to set MFA requirements and enforce them even if the clients themselves don't require that in their Azure Tenants (which they won't if they are created on the fly).
The downside is I believe B2B is still considered preview, so thing may change on you.
After you figure out the Auth, the other piece that may make sense in your scenario may be using Office 365 Groups. You can have one for each client and you can still leverage a central SharePoint site and Pin that to your groups as you create them.
I don't know about forwarding - but you could consider creating custom app tiles for each customer to their Office 365 groups page, and assigning those to only the users that need the specific tiles. That way those specific sites appear as tiles in the app launcher but only for the users assigned them.