I think creation of a SPO site should be a flow trigger.
We would use the trigger to then flow as an item into a SP list, that records links to all subsites within a site.
Right now (to my knowledge) such a "list" can only be done using the TOC web part (which is extermemly limited in functionality), or a CSWP (which is extremely limited in display options).
The list of subsites functionality was apparently removed after SP 2007/2010 versions. Now, if you do not know coding, and want something OOTB, we should have the ability to do this in SPO.
Thanks for the ideas.
Please consider submit this as an idea in the Microsoft Flow idea forum:
Currently Microsoft Flow would support to work with SharePoint list, SharePoint Files, and there are planning for support SharePoint library:
You may also consider to use SharePoint API to define a trigger by yourself:
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