12v should be decoupled (say, 10 uF -low pass, for relay activation- + 100 nf).
3.3v decoupling (2*0.1 uF : 100nf + 0.1uf) is either a typo, or unsufficient (100 nF + 1 uF would be better)

Is your uln2803 + relay decoupled? (see http://forum.arduino.cc/index.php?topic=157018.0)

One weird behaviour is, when I put my hands closer to the DPS, the number of wrong values are greatly reduced. Maybe this is the reason of my problems, but I do not know what to do ... Seems a symptom of your problems. You really should post a (link to a) photo of the circuit as it is: a)Are each a...

Two things keep on worrying me (searching for powwer supply trouble):
a) you have an analog circuit before the analog to I2C converter. Is it decoupled? How is it powered?
b) ESP have separated flash (another chip), with a separate decoupling (say, 1uF).
c) Is your ESP32 tied to an antenna?

Well, your ADC has a 100 n decoupling ;OK I2C legs are pulled up with 10k ;OK EPS32 has ... two 100 nF decoupling (200 nF) From https://www.espressif.com/sites/default/files/documentation/esp32_hardware_design_guidelines_en.pdf , fig.2 -maybe 3- seems it lacks >= 1uF decoupling capacitors . from the...

Well, my answer wo not solve your problem, but might avoid troubles . I saw some classical troubles in your main.cpp: line 13 : What is the use of delay(1000) ; lines 28..34 : what happens if there are more than two lines? I would have written (more concise; safer; easier to understand... but untest...

Other issues might be pullup resistors (SDA and SCL should be tied to + via a , say, 2.2k resistor, else, it gets noise sensitive -and as ESP_Sprite wrote, wifi can generate noise). Is there a way to put pictures of your hardware in this forum -on both sides, so that nothing gets hidden: "smart" "ph...

Excuse me, but if you are under linux/Unix, ln -s can be tryied (shares (contents of) a file between two directories (file names): if you modify one version of the file, say in directory A, it is modified in the other directory and no extra room (except for a "adress" ) is used.... This can be used ...

you should not worry that much about optimizing resources, as ESP32 are way bigger and faster than 8 bits arduino. Both String and char arrays are comfortable for me, and conversion is very fast using .c_str (ask arduino help : is in your IDE): creates a pointer, no RAM displacement (very fast); toC...