Hello,
1. Design you’re a landing zone - put all subscriptions under one roof by leveraging management groups if you own them all. Other option is to build vnet peerings between subscriptions
2. Collect both business and technical requirements
2. Use workshops with stakeholders to showcase the future state of the environment then use it's outcome to design the final solutions (principle: design before you build else someone may ask: what are you building?) . The design should cover all stated requirements including the need to establish a hub and spoke topology in order for you to utilize all the subscriptions you mentioned for whatever resources you want to deploy in them.
note: If you don't want to maintain domain controllers as VMs, with a budget of about $150/Mo, you can spin out an Azure AD Domain services which is a platform and has build in redundancy.