I'm not sure that seccomp is enabled in systemd. cpfpd opens and closes its configuration file, starts a server for the requests, opens a socket to the control port... That's a lot of system calls. To be checked.
Perhaps nothing new to you, but Firejail Seccomp looks like an good contender in this field. It was added very recently to the Debian FTP archive (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=777671). Available in sid only, not in Debian stretch / testing.
From the firs look: amongst some features, it uses secomp sytem call filters (default blacklist, can be expanded or used along a whitelist), it's configurable with profiles, there is a --private option that mounts /home and /root in in a tmp file system, destroyed when the sanbox is closed... AND it's compatible with AppArmor.
It's very easy to use. The browser I'm using to write this was started with
~/tor-browser_en-US/Browser$ firejail --seccomp --debug ./start-tor-browser
Tor Browser is still contained by AppArmor.
Using start-tor-browser and not start-tor-browser.desktop because the latter is detached and the sandbox parent process terminates.
The --debug option gives a fair idea of what the default installed version is doing.