Sharing with External User/Distribution Groups

Hi, looking for some direction on sharing our content with a large number of people in an external org.

We publish a report that our client wants to be accessible to hundreds of their users. In this case everyone is using PowerBI Pro on both sides.

As a small company ourselves, we don't want to individually administer hundreds of access requests. So we're trying to identify a way we can allow the client to maintain access to that report/resource. (we're flexible over whether this is a dashboard level, workspace, app etc). They do not need the ability to edit anything, just consume.

We initially tried 'share with reshare access', but reshare only allows access to be granted within the host org, so this wasn't helpful.

I've been experimenting with using groups, mostly using my personal tenancy (personal_org.com). I was able to create a group (data@personal_org.com), and give that group access to the resource. By instead using a Mail Security Group (sec-data@personal_org.com), I was able to also add an external user ([email protected]), and was, therefore, able to access that resource as a guest. But that still leaves us (persona_org.com) maintaining the group.

If I hop back to my work tenancy (work.com), and share a report with my personal tenancy via the same security group email address (sec-data@personal_org.com), I get a generic 'failed to share' message back. That suggests that the host tenancy can add a security group (that in turn has external users), but you can't add an external org's security group. But maybe there's a security setting on the tenancy I don't have access to, nor know to ask to be changed.

So, is this something that is even possible? Ideally, I'm looking to add a group that is either externally owned to at least externally administered. They can then decide who gets access - and/or they can create a dynamic group.

(I know embedded is another route, but as the client already use powerbi they're keen to have it somewhat integrated into their existing space)