Unfortunately you can’t make an Ubuntu system into an Arch based one. Both of these use totally different package managers, but I’ve heard about a distro which lets you use apt (ubuntu, debian), dnf (fedora), pacman (arch) package managers in one place, but haven’t checked it out.
Try out an encrypted desktop on a VM if you can. Apparently you will need to remember 1 additional password whenever you wish to use that encrypted storage, but it’s totally valid if you need an encryption.
Creating a DE by yourself sounds like a madman task (especially if it’s not based on an extising one) but I’m rooting for you! You can also help an existing DE on their respected repository.