According to these, did you manage to switch from RST to AHCI in the BIOS without reinstalling Windows? This is good news. Did you resize your Windows partition with Windows or Gparted? In the former case, after the first Windows boot after installing Linux, you can expect that the machine will most likely start with a file system scan. Here is a good EndeavourOS installation guide with dual boot boot.
In response to the pm you wrote to me: Are you sure you did not forget to turn on the boot flag for the EFI partition?