Hysteresis really should be straight forward for someone familiar with embedded technologies but LOL, think on that introspective discovery a bit. You are unknowingly scratching on some hidden wounds which individuals experienced in the field tend to avoid. That would be an example trigger phrase.
This is a starting point, if it has a micro-controller involved you’ll have 50+ percent of the work that could be built upon or ported to another controller.
Also a start. A well architected system is modular. Which means you can strip features from the software that you do not need. If you can determine an open source project that is using off-the shelf DIY modules such as arduinos, Atlas shields, and so forth, you’ll have little to do beyond stripping of the un-needed features. Maybe a little bit of figuring and programming for anything unique beyond what is provided. And, of course, getting up to speed on how to use the “stuff”.
If you are doing PH, you’ll want to measure temperature as well for compensation. So, either two probes (temperature and PH) or a probe with a temperature output, too.
And, what @lefthandseeds has said. A PH probe with a constant service/immersion rating if the probe is going to remain in solution for long periods.
See Leaving the pH probe in RDWC reservoir...?