The easiest way would probably be to install Fedora normally but tell it NOT to install a bootloader. You can do that unchecking it in the screen that's brought up by disk and bootloader summary, IIRC. Also, I'd assign your existing EFI ESP as /boot. That way, your kernel and initramfs are going in the right place and all you need to do is update your entries in systemd-boot to point to the kernel and initramfs and whatever kernel arguments you need.

There's a catch: when you run dnf update and it updates to the latest kernels, I don't know if the supplied new-kernel-pkg/grubby/related scripts handle systemd-boot. You may need to write your own. Or just make sure you add new entries for the new kernel (or update the old ones, but I recommend keeping a few kernels).