The width of the motherboard seems to conform to ITX size.
But, the depth of the motherboard is more like eatx depth.
Here is a link to ATX specs and dimensions: https://en.wikipedia.org/wiki/ATX
By measuring your motherboard and comparing to a prospective case, you should be able to find something that fits.
The dimensions are 13.8 x 13.4 x 6.4 inches (350.5 x 340.4 x 162.6mm)
The motherboard is proprietary and the extension is designed to mate with the front ports of the case.
If you were to mount the motherboard in a EATX case which is deeper, you would not be able to mate the front panel outputs to any other case.
If you want a better case, the easiest thing to do is to buy a replacement motherboard to go with the new case.
Most any lga1151 300 series will do for your i7-8700 processor.
Here is an example for $60. https://www.newegg.com/p/N82E16813157952