Hello @Tom Stewart,
the Microsoft 365 E5 Developer subscription has the same licensing and features as a regular Microsoft 365 E5 subscription, except that inbound connectors for mailflow are not supported in Exchange Online and you cannot purchase additional services through your Client Administrator account. Any additional services (e.g. Azure) must be purchased using your personal/Enterprise account (e.g. {myusername}@{myorg}.com instead of admin@{mytestdomain}.onmicrosoft.com) and then moved to the subscription.
If this is helpful, please accept the answer. Thank you.