Another C1246 ABS thread - journaling and troubleshooting

My 2010 Prius lit up all the ABS lights and threw an error code (I can't remember which) so I had the actuator and accumulator replaced with used parts by a local shop. After 3 months, the lights came back and an error code showed up, but it was still braking just fine (just a little squishier, but very reliably).

I drove it for about 6 months like that, topping off brake fluid every few months, then suddenly the brake resevoir was empty and braking was as bad as you would expect. I fortunately had the brakes go out less than a block from an auto parts store, so I pulled in and topped off the brake fluid and drove around the parking lot for a while before driving home.

The brakes never went back to normal, but if I lightly pump the break once then press again, it brakes as expected. After leaving the car parked for a week (definitely not driving with the brakes activley misbehaving) and checking the cardboard I put under it, there were no signs of slow leaking so I read the error codes with the "Car Scanner" app and Veepeak Bluetooth adapter and got C1246
I reset the ABS errors and drove the car around the block a few times and it brakes very quickly, basically the way it did when we first got the prius a few years ago. (I'm still not driving this on open roads)

Because the brake fluid drained, I know I have a leak and it isn't just a sensor issue.

The rattling of the actuator seems normal, it buids pressure and holds, and after a while builds pressure again as I use the brakes. My older gen-2 prius had the actuator rattling all the time, which made me think the accumulator was leaking, but we still replaced accumulator and actuator with used parts and it is still holding strong 5 years later.

I'm mainly posting here to see what others' experience has been with this error code, and to document my journey for any future issues.

My plan now is to get quotes from my original mechanic and a chain like brake masters for repairs with new parts. I also don't want to pay $800 to $1500 in labor to replace just one part if there's a solid chance I'll need to replace the other one later, but I also don't want to spend an extra $800 for something that is not likely to actually fail.

I've removed the top cowl and have full access to everything, but I can't see where fluid is actually exiting the assembly (I assume it is an internal leak, but I'd love to rule out simply needing to tighten a brake line).

Any tips other than just biting the bullet and paying a mechanic to replace both?