Unfortunately you are far from the first person to have this happen. Typically when this occurs the frame is a write-off. The costs and the risks associated with fixing it will exceed the costs of starting fresh with a frame that is not compromised. Even on steel frames where the dropouts just spread... they're never the same afterward.

Its a bitter pill to swallow but there it is. Before the next one gets built, research torque arms and find a way to use two of them on a big motor. Its by no means an issue to build a bike with a powerful hub motor but a strong torque arm solution is part of the cost of admission.

Also... it looks like you filed the dropouts to get a better axle fit. I can see the edges front and top of what is left are squared off. Its not an uncommon move especially for 14mm axles that are way taller than the dropout depth. But that introduces a classic weak spot on a frame that - over and above the torque arm issue - can cause exactly what we see here. In fact it looks like what you suffered was not a spinout but an outright snap since the front of the dropout is pretty well intact - no gouge where one half of the axle dug in like you usually see. Seems like the back half just broke off before a gouge could occur.

If you are going to go big on power, in addition to torque arms... consider a steel frame with steel dropouts.