SQL Managed Instance - Cross Subscription Restores/Migration

Cross-subscription PITR restore for the same region and same Tenant ID (e.g.same customer/company only) is available as a limited preview for EA, CSP or PayGo subscriptions only at this time (April 2020). You would need to login into Azure portal (through EA/CSP/PayGo subscription) where you intend to use this and create a support ticket, selecting support option "Automated backups or point in time restore". In a few days you could get approved for the limited preview. This solution also supports TDE encrypted backups for Azure managed keys. For BYOK keys this will work only if customer has the same key on both source and destination MI. Hope this helps.

copy only backups are currently the only solution. Cross subscription pitr and geo restore don't work because currently we cannot verify that initiator has all required permissions both on source and target subscriptions.

To add to Jovanpop's answer, you should still be able to take copy_only backups with TDE turned on if you are using AKV integration i.e. BYOK TDE, as long as your destination instance in other subscription has access to the same encryption key used on source instance.