So, here we are trying to do the transition. I followed the detailed guide provided by Gentoo on how to install systemd. After I finally rebooted, everything was working as expected, beside one little, small, annoying thing: the boot was taking 90 seconds.
So I inspected the log and find out the following messages during the boot:
*A start job is running for sys-subsystem-net-devices-multi-user.device
Apparently the service email@example.com was not enough. Something was using a multi-user device.
With a little help from my friend grep I found out the culprit.
grep -RHn multi /etc/systemd/
While the aforementioned file was referring to the multi-user device, the file /firstname.lastname@example.org looked good.
I hardcoded the interface in the mentioned file, in vim you can simply do :%s/%i/wlo1/g and at the next reboot, the boot was quick as one would expect.
Probably making the wpa_supplicant@.service a symlink to email@example.com would have sufficed as well.
NOTE: I only had firstname.lastname@example.org enabled with systemctl enable email@example.com.