Pihole + Unbound = reply error is SERVFAIL

Dont need to have a cron job updating the root.hints.
When install unbound via the APT packaging system, it already comes with root.hints:

pi@ph5:~ $ apt depends unbound
[..]
  Depends: dns-root-data
[..]

pi@ph5:~ $ dpkg -L dns-root-data
[..]
/usr/share/dns/root.hints
[..]

root.hints doesn't change that often so you just have to keep your distro updated to also update the root.hints:

sudo apt update && sudo apt upgrade

Also opening 80/443 to the public is not recommended for security reasons.
While the Pi-hole web GUI is safe to run at home, it isnt hardened against attacks etc when exposed to the Internet.

Yes is required.
Not because of unbound but because systemd-resolved is also a caching DNS forwarder same as Pi-hole.
The two would conflict fighting over ports 53 UDP + TCP if dont disable systemd-resolved:

pi@ph5:~ $ sudo netstat -nltup | grep 'Proto\|pihole-FTL '
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:53              0.0.0.0:*               LISTEN      2207/pihole-FTL
tcp        0      0 127.0.0.1:4711          0.0.0.0:*               LISTEN      2207/pihole-FTL
tcp6       0      0 :::53                   :::*                    LISTEN      2207/pihole-FTL
tcp6       0      0 ::1:4711                :::*                    LISTEN      2207/pihole-FTL
udp        0      0 0.0.0.0:53              0.0.0.0:*                           2207/pihole-FTL
udp6       0      0 :::53                   :::*                                2207/pihole-FTL

Did you alter /etc/systemd/resolved.conf and why ?

Is date/time in sync/not too far of ?

timedatectl

What do below two output from the official unbound guide ?

dig +noall +comments @127.0.0.1 -p 5335 sigfail.verteiltesysteme.net

dig +noall +comments +answer @127.0.0.1 -p 5335 sigok.verteiltesysteme.net

To up verbosity for the loggings and display on screen, you could stop unbound:

sudo service unbound stop

Run here manually with below:

sudo /usr/sbin/unbound -ddd -vvv -c /etc/unbound/unbound.conf

Run the two dig commands again posted above and take note of any errors/warnings.

You can stop unbound again by pressing CTRL-C.
And start here up again using systemd:

sudo service unbound start