...what makes sense to attempt to repair at operator level, for both airline and bizjet.
Apart perhaps from wheels & brakes and batteries, nothing.
That's right, nothing at all.
The answer is "Supply Chain Management" and contracts with specialist companies that stockpile components, do all the repairs/overhaul and provide consignment spares for all the Code 1 No-Go items, backed by 24 hour support for the Codes 2 and 3. There's plenty of these companies around and they're spreading fast. We have millions tied up in inventory and repair shops. Flogging it all off to a supply chain manager and converting the No Goes to consignment frees up a lot of cash. The service is paid for from operating revenue and you get guaranteed TAT into the bargain. In economic terms its called "Economy of Scale".