If it were me I would make sure secure boot is off as you don’t need it for Windows to work or Ubuntu and then you could install refind on EndeavourOS and use it as the boot manager. It gives you the option to boot from the grubx64.efi or the vmlinuz-linux image file. When it boots it boots to refind and then you can access Windows or Ubuntu or EOS from an Icon. Really it’s your choice. You can just use grub also and boot any OS from a grub menu. Install EOS last and let it control the boot. Then if you decide you can install refind to it and then it will be the boot manager. refind is very simple to install. Best thing is to disable os-prober on Ubunto so it doesn’t get updated and take over the boot. Then if and when you decide to use refind you should have no problems. I have triple boot myself using refind.
I myself use swap files vs swap partitions.