Is losing power when pulling Shimano GRX lever a bleed problem?
The symptoms you are describing seems to be a &quot;Ghost Leak&quot; - Rides of Japan terms it this way. It might actually just be a kink in the hydraulic hose.
His symptoms are similar. Squeezing the brake feels powerful, but holding on to it will eventually cause the hydraulic pressure to release. Once he lets go of the brake lever, and squeeze it again, he can feel the power again.
Definitely recommend you checking out Rides of Japan's recent video on this, it's quite in-depth as he shows why it happens.

                
            
In short, over a period of time, the points where the hydraulic hoses get kinked will wear out over time. Especially if there are accessories like handlebar bags there actually slightly re-route the hoses' entry into the frame/cable guide. As the hoses are under pressure from the hydraulic fluids, eventually the inner lining will disintegrate, causing a kink.
In your case, could it be that the hoses actually got hit in the accident? It might be ideal to replace the hoses too! If really there's a kink in the hose, it won't be apparent from the outside too.
Rides of Japan is quite thorough for this issue (for a home-mechanic at least), it's easy to understand from his documentation! Hopefully, this is the correct approach to your issue, good luck!
P.S. I mentioned Rides of Japan like 3 times, but damn it his videos are binge-able!