Finding Loop Gain (Aβ) of Transimpedance Amplifier
There are two important conditions for finding a "correct" breakpoint:


The dc operating point of the circuit must not be influenced. That means: For circuits with REAL opamps there must be still a DC stabilizing negative feedback loop. During simulation and for an IDEAL opamp this requirement can be neglected.
The loading at the opening must not change (in comparison to closed-loop operations). Hence, the opening must therefore be directly at the opamp output (zero output resistance assumed). Otherwise one must apply alternative methods, which are available (mirroring of the load,...). However, in your case, this condition is not relevant (because of the next point).
In the present case - assuming an ideal voltage-in and voltage-out amplifier with gain A - the loop gain is identical to the open-loop gain "-A" of the amplifier (100% voltage feedback) because there is no current or voltage drop caused by the test voltage.