Hi @felix-oehl
Completely agree with @pswider @Expiscornovus, Creating a site template is not yet available. You can only do page templates and web templates.
Also, provisioning each site when they signup is a good idea. Below are some of the questions for you
1. Did you consider the pricing for each portal?
2. Are you happy to accommodate internal and external users pricing charges?
3. Power Pages is not yet GA. If you take portal pricing as an example still it is high.
4. PAYG is an option but still, will it be worth for ROI?
Alternatively, you can have one PowerPages / Portal then when the customer signs up then you can customise the home page / modify power BI reports by applying RLS and create custom web roles to show different pages according to their needs. You can even do dynamic data and images depends on the web role. This approach is a huge cost saver.
I haven't tested the Power Platform CLI and PowerShell commands fully yet. But your ask maybe viable but not tested yet.
Please think about the above-mentioned scenario and see if it helps.
There are deep topics which have to be discussed but for now, the above questions will give you some clarity.
Hope it helps.
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