With a RPI4 the setup is very simple just add to /boot/config.txt :

enable_uart=1
dtoverlay=pps-gpio,gpiopin=23

And the PPS interrupt is working :

pi@raspberrypi:~ $ sudo ppstest /dev/pps0
trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1569142530.000035957, sequence: 125 - clear  0.000000000, sequence: 0
source 0 - assert 1569142531.000034676, sequence: 126 - clear  0.000000000, sequence: 0
source 0 - assert 1569142532.000033663, sequence: 127 - clear  0.000000000, sequence: 0

To setup the NTP server look at : http://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html 

The enclosure is a PVC pipe adapter I found at a local store for 1.2 Euros :

Once it's closed it looks good :

The top is screwed and it has an inside seal, I just needed to cut a piece of plastic to fill the void.

I'm running a test with a 100 meters (328 feet) ethernet cable between the RPI and the GPS head and it works without a hitch. It should work with longer length without problem, the limiting factor should be the voltage drop but at 24V the current is small, and one pair in the cable is used for GND and another for +24V.

Basically the project is finished and it's working great. I need to publish the schematics and the gerber for the boards and add a step by step howto to ease the replication process.

On problem though: It is my first project with a RPI4,and the board is running really hot. I'm still waiting for my enclosure but there is no way the processor will run full speed without throttling when the hat is on and without heat sink nor fan. So I will make a new HAT with a fan... In this version I have a RTC with a backup battery, it works great but I do not think it's really useful in real life. So in the next version I'll remove the RTC to make room for a fan and I'll add few LED to show the status of the NTP server.