Modern subsites?

I am having the exact same issue. I am in the process of trying to plan for migration and want to migrate from on premise classic to cloud modern. I think mixed use will just bring so much confusion to my user base. I have a definite need to have shared lookup columns also (ie Customer lookup etc). It is crazy to have to maintain multiple lookup lists what a mess. I use AD groups for security so I don't need O365 groups for security, however I do use SharePoint groups only for SharePoint Designer workflows and sometimes for field level security on custom forms using client-side JS. I am hoping that I will be able to use my AD groups for workflows when I rewrite workflows using Flow. I have no idea whether I can use the AD groups for custom forms using either PowerApps or SharePoint Framework custom forms.