Is the subpanel installation in a detached garage correctly done?
So, you have 8 gauge wire potentially subject to a fault current of up to 200 amps without anything tripping. You also have the neutral and ground bonded in a sub-panel (despite running separate neutral and ground wires to it, so bonding in the sub-panel is a puzzling choice in itself, as well as being a violation.) [or else possibly you have written unclearly about what you have done with the ground and neutral?]

Wrong. Just....wrong.

Pull two breakers, insert a 50 A breaker, and move the 15A circuits to the sub. And unbond/isolate the neutral and ground in the sub. Get a different/larger sub-panel if you need additional circuits (or give a long hard look at a complete panel replacement with adequate spaces in the main panel - not all that expensive if the incoming service is adequate, but attached to a panel with few spaces.)

If your panel is listed for them breakers with two separate 15A circuits in a single space breaker are another option to make space.