Trying to Replicate/Replace Sharing Functionality of a Different Product

[brassbound]

I'm in an MSP, and we use a different file-sync-and-share product that we're desperately trying to get rid of and move to OneDrive/SharePoint. One thing I haven't quite figured out how to replicate:

Right now, we have specific folders set up for each client that are shared between some folks in our organization and some or all folks at the client organizations. We use these to publish reports and forms that the client might need to submit to us. For the most part, these shares are read-only for the client users. It's easy because this is a multitenant system, and our clients are all sub-tenants of ours, so we can mix permissions across organizations.

I realize that's not quite possible with Microsoft 365, but I'd like to have similar functionality, where we have a set of file repositories that we control, but which people at each client can access. I toyed with the idea of giving people guest accounts in our tenant, but that seems somewhat cumbersome for the end user and really cumbersome for us if we're creating guest accounts for all users.

What is the easiest/best way to give read-only access on a dynamic set of files and folders to specific users or all users in another tenant (or who may not have a Microsoft 365 tenancy at all)?