Along the same lines as Will's suggestion, you could try just inverting the Y-axis velocity, or explicitly set it to a value.

The scripting reference for RigidBody.Velocity suggests:

In most cases you should not modify the velocity directly, as this can result in unrealistic behaviour

However, that may not be an issue for you here (depending on style of game, and the gameplay outcomes, etc)

If you want to rebound based on how fast you are falling, you could try something like:

rigidbody.velocity.y = -rigidbody.velocity.y * 0.8;

This would mean that if the player character was moving (down) on the y axis at 10 m/s, they would be instantly set to bounce up at 8 m/s.

If you wanted reliable bounces, you could explicitly set it to a value.

** This may not be the best way, but I don't think using the relativeVelocity is ideal for typical 2.5D platformer behaviour.

Well I am new too, but have you tried finding the velocity of the player and then on collision reversing it into a negative?
on collision{ rigidbody.velocity.y = (collision.relativeVelocity-(collision.relativeVelocity*2)) rigidbody.velocity.x = (collision.relativeVelocity-(collision.relativeVelocity*2)) } No idea if this would work or not but it might be worth a try.