Have tried again (against developer tenant) with Graph powershell to create a new team based on 'custom template'. I would assume that changing 'standard' in this part of code to new custom template name should be sufficient:
"template@odata.bind" = "https://graph.microsoft.com/beta/teamsTemplates('standard')"
But so far no luck. Using the 'name' of the custom template was not going to work. Also couldn't find any documentation (yet).
With new templates that were provisoned by Microsoft the ID of template has format like 'com.microsoft.teams.template.AdoptOffice365'. Using this name spaced name: perfectly able to provison a new Team based (using Graph PowerShell).
My new copy of a template only gets a GUID, using this GUID results in 'template@odata.bind' -> seems to do the trick. Was able to provision new team based on customized teams template. Takes however some time (minutes) between apperance of new Team in Teams client and stappeling of changes. Also (per accident) ran new team Graph call with template against already provisioned Team, it created additions of template in that Team.