Why not give it a try?
V6 is dropping the lighttpd/php dependency anyway in favor of a web deamon (Civitweb) embedded in the pihole-FTL binary itself:
$ sudo ss -nltup | grep pihole-FTL
udp UNCONN 0 0 0.0.0.0:53 0.0.0.0:* users:(("pihole-FTL",pid=1880538,fd=20))
udp UNCONN 0 0 0.0.0.0:123 0.0.0.0:* users:(("pihole-FTL",pid=1880538,fd=40))
udp UNCONN 0 0 [::]:53 [::]:* users:(("pihole-FTL",pid=1880538,fd=22))
udp UNCONN 0 0 [::]:123 [::]:* users:(("pihole-FTL",pid=1880538,fd=41))
tcp LISTEN 0 200 0.0.0.0:443 0.0.0.0:* users:(("pihole-FTL",pid=1880538,fd=37))
tcp LISTEN 0 32 0.0.0.0:53 0.0.0.0:* users:(("pihole-FTL",pid=1880538,fd=21))
tcp LISTEN 0 200 0.0.0.0:80 0.0.0.0:* users:(("pihole-FTL",pid=1880538,fd=33))
tcp LISTEN 0 200 [::]:443 [::]:* users:(("pihole-FTL",pid=1880538,fd=38))
tcp LISTEN 0 32 [::]:53 [::]:* users:(("pihole-FTL",pid=1880538,fd=23))
tcp LISTEN 0 200 [::]:80 [::]:* users:(("pihole-FTL",pid=1880538,fd=36))
After you've installed v5 and before switching to the v6 development branch (checkout), dont forget to disable and stop lighttpd first with below if you dont use it for anything else:
sudo systemctl disable --now lighttpd.service
If not, lighttpd will keep on running after the switch and the pihole-FTL daemon will be configured to listen on port 8080 for HTTP instead of 80 where lighttpd will still be listening!
Am not sure if pihole checkout development is sufficient to switch.
But am sure @DanSchaper can tell 
EDIT: Oh blast, after typing above, I realised there is no armel pre-compiled binary available for your Pogo: