Do I really need a new engine control unit? Or is there a way around this?

No. I have no idea why the report suggests to “Replace Engine Control Module (ECM),” since the diagnostic trouble codes (DTCs) listed (C1391, U0073, U0123, U0124, U0126, and U0293) all came from the brake system, and Toyota’s Repair Manual (more info) doesn’t identify the ECM as a potential trouble area for any of them.

The procedures involving the TS, TC, and CG terminals can help diagnose a problem, or clear the stored DTCs after repairs have been made, but they’ll never fix anything on their own.

That depends on the results of a proper diagnosis.

The Uxxxx DTCs all indicate that the skid control ECU (the computer that controls the brake system, built in to the brake booster with master cylinder) couldn’t communicate with various other devices over the CAN No. 1 bus. Since a simultaneous failure of several such devices—the yaw rate and acceleration sensor (U0123), the steering angle sensor (U0126), and the power management control ECU (U0293)—is highly unlikely, it’s more reasonable to suspect a problem with the CAN wiring or, perhaps, with the skid control ECU itself.

To start, however, I’d focus on DTC C1391, Accumulator Leak Malfunction, for which the Repair Manual procedure says to check for brake fluid leaks and dragging brakes, and if that’s not the problem, to do further steps to determine whether to replace the brake booster with master cylinder or the brake booster pump.

There are many other PriusChat threads about C1391, covered under Toyota’s Customer Support Program ZJB (PDF), but now only “for 10 years from the date of first use or 150,000 miles, whichever occurs first.”

 

Also, in a Gen 3, the skid control ECU is part of that "brake booster with master cylinder" assembly, so if diagnosing the C1391 leads to replacing that assembly, you'll already have a new ECU as well, so then you'll find out if those communication errors were also the fault of the ECU, without having to work at it.