see the table below:
Source - https://www.parallels.com/blogs/ras/microsoft-vdi-licensing/
As you've seen, per-user is only really relevant for external users if you do not have the relevant on-premises or M365 E3/E5/Business Premium licensing:
I would echo what @Andreas Baumgarten has said above - make sure you either contact Azure Sales directly or else engage with your licensing partner or CSP to answer your queries and make sure you are fully covered for what you want to do on AVD.
Hope this helps,
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