Putting diodes in parallel is not a great idea, but adding series resistors to help balance them will help. I would recommend using a single diode
that can handle thge required current.
Connecting a transistor with base to collector will act like a diode, but the reverse voltage rating will be only about 6.8 volts. The 6.8 volts is a generalization, but that is typical of the base emitter junction.
How about putting a high voltage, low amps diode in series with the base pin, will that help the base-emitter junction? Will it also cause the transistor now to act as a diode but with 1.2V drop instead of the nominal 0.6V (at low currents) ? I am not at home to try this on the simulation just now.
I do not know why you are having a hard time finding diodes greater than 6A, go to the phone book and look up a company like semikron or westcode, There are 3 westcode 1800A 1600V diodes on top of my fridge.
For smaller stuff the MUR8xx and RUR15XX TO220 package diodes are common getting larger the stud mount DO packages start at 10A and go around 400A Unfortunately they get expensive
Bridges in 25 35 and 50 amp are common and cheap 100A are available but assembling from 2 diode modules is popular in this current range.
Diodes are used in parallel and they share current if matched, that is how rectifiers are built to handle 10's of kA.