Since I know myself in the pool I have several things to say:
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Priority to filtration. the quality and ease of maintenance of a swimming pool depends on filtration. As has been said, the small pumps provided with these pools are "gadget" the filter clogs very quickly and harms the quality and renewal of water don its quality = change the pump by a sand filter system and small pump like this one for example:
https://www.intex.fr/shop/filtration/3564-filtre-a-sable-4m3-h--6941057415499.html. -
Treatment (measurement and disinfection) and filtration must be done during the day: bather + sun = water pollution. At night, not much happens and the sun does not destroy the chlorine present in the pool.
Yes, the pH does not move much once balanced between 7.2 and 7.4 BUT after filling the pool it will go up very quickly (at least here in France hard water so high pH). -
To start and take your first steps in the pool: you need to measure the chlorine and the pH with strips or drops like this:
https://www.amazon.fr/Manufacturas-Gre-Testeur-hydromassage-Expédition/dp/B07PB2LX2Y
a float like this: https: //www.amazon.fr/Distributeur-thermomètre-applacteur-automatique-désinfection/dp/B07SVYWDVX
: you put a pebble of chlorine in it and let it diffuse. There's even the built-in thermometer!
and a bucket of pH corrector + or - powder, according to the evolution of the measurements and your pH of the water.
And above all, clean your sand filter once a week by acting on the backwash valve of your superb filtration.
You will be able to see how the water in your pool evolves day after day. -
To go further: Node-red? Yes, you could do everything with Node-red: pH, temp, redox, and more ...
Each pH and redox probe measures milivots. It is enough to transform the mV into pH according to a curve. The redox will be the measured value ex: 600mv is a correct disinfection rate on average. It all depends on the mV of your original filling water.
CAUTION: as read here, certain probes have an electrical link with water: pH and redox measure the mv in water, so they must be isolated from their mass. Either using an isolation circuit (power and signal), or make an electronic card with several transformers. A Transfo (230vac => 5vcc) for the Redox function and a Transfo (230vac => 5vcc) for the pH and Temerature function. As such, the power supply to the circuit and the measurement will be isolated from each other if they are in the same liquid.
This is THE delicate part of the project!
The temperature probe is electrically isolated from the water so no need for an insulation circuit. I recommend a cheap, waterproof DS18B20 sensor probe, in "one wire" very easy to connect to the RPI (as you have already seen):
Display temp (18b20) on oled with just pi and nodered
You could already start with this is the simplest. -
Which sensors to buy: expensive, not expensive? No matter, they are all made in China lol It is not because it is more expensive that it will last longer ....I have less scruples to renew every year, a pH probe at 10 € than a probe at 50 €
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Where to place them? : On the piping, in line after the sand filter with a bypass system as you have already shown, it is ideal.
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Dosing pumps: https://www.amazon.fr/doseuse-peristaltica-liquides-membrane-santoprene/dp/B075HXSMN8
pumps of this type are very good: injection of acid (pH-) or base (pH +) or liquid chlorine, works at 230vac, so you need control relays controlled by your NR. -
The software: these pumps have a flow rate of about one drop per second (1.2l / h)! No need to make a complicated program:
Ex: if you give a pH set point at 7.2, and your measured pH is at 7.3 you start injecting ph-liquid (20L can or less, you will see), as soon as your measurement is at 7.2 you stop d 'inject. You will see later if you need to add timers to limit the duration of pumping of liquid, but not necessary.
Investing in quality filtration guarantees you ideal water quality and product regulation.
Even if the suction and discharge of your filtration are not very far apart.
It's already been a lot, good tasting.
[EDIT] I forgot an important thing @BartButenaers : think of leveling the ground in general they recommend to put a layer of sand: Do not forget that you have 12 Tons of water in your garden ! (look at your water meter before and after filling will give you an idea of the volume).
And protect the bottom of your pool against peaks that could puncture your waterproofing, a leak would add more maintenance concerns 
