Borderline impossible if you aren’t using a distro designed with that in mind. Pretty much everything that isn’t a program you directly start (e.g. sound system, desktop environment, bluetooth daemon etc.) either only provide a systemd unit to start them (which you’ll have to manually translate into e.g. a shell script if you want it to work with your new init system) or is entirely reliant on systemd to function.
Your choices of distro if you don’t want systemd are Debian, Void, Artix, and Gentoo, and afaik that’s about it.
Replacing components of the systemd suite (e.g. using connman or networkmanager instead of systemd-networkd) isn’t actually that bad as long as your DE has support for them, but replacing systemd itself is something you are building your entire system around.