Agree this would be fantastic, not being able to clone to portals is not good. I have to manually recreate intricate flows that I painstakingly developed on our test development server. Plus when expanding, as the OP suggests, quite often using flows on different projects can be very useful.
Klaviyo (Different Platform for Email) has very intricate flows that HubSpot can replace and in Klaviyo I can clone from one portal to the next very easily. HubSpot Designer Tools even has Clone to Portal available!