Is your system with Secure Boot? If so, the kernel doesn't boot because it's not signed by the key that is official for distribution. Other than that, I can think of the inconsistency with Nvidia binary driver. Maybe we should wait for the official release of 5.7 TW kernel, then. So, uninstall kernel-default-5.7 again, and go back to 5.6.y. Do you have any special boot options? Please give the dmesg output after the boot.