Is it possible to allow sideloading and keep users safe?

It's not sideloading, you are not doing anything nefarious,shady, on the side, on the edge. It's software installation on your device, your own device. This newspeak is purposely invented to negatively portrait software installation from sources not controlled by Google/Apple

The term side loading pre-dates smartphones. The term was used to describe how you got media onto an electronic player. Literally by plugging into a port on the side and loading the media from a computer.