(although the uC could also be a CH32V003)
It's almost certainly not - as the linked page suggests - an STM8, nor a CH32V. It's probably one of any number of STM8S003 pin-compatible asian-brand microcontrollers that are typically 8051 or Cortex M0+.
Looking at the reverse-engineered schematic on the linked page, one thing stands out to me that can dismiss the STM8 and CH32V, and possibly narrow down the potential candidates: that there are
two signal lines from the microcontroller that go to the programming header. Neither the STM8S nor CH32V003 need this - they each only have a single data line for programming. In fact, an STM8S could not work in the configuration shown, because it requires a capacitor on the VCAP pin for the internal voltage regulator (CH32V does not have such a thing; pin 8 is a regular GPIO).
One microcontroller that I am aware of that has a TSSOP-20 '003 compatible pin-out and that has programming signals on pins 8 and 18 is the
WHXY CW32F003F4P7 ARM Cortex M0+ micro. Pin 8 is SWDIO and pin 18 is SWCLK. It's probably not this exact chip, but some other that also has clock/data programming lines on those pins.
By the way, a
datasheet for the SZ2525C was easy to find on LCSC.
Edit: I think I may have found a candidate for the MCU.
The
Wisesun WS51F0030. It's an 8051-based microcontroller. Has the programming clock/data pins in the aforementioned locations. Also runs on 5V (some ARM MCUs are not a match as they only run on 3.3V). And, according to the pictures on LCSC at least, has no top markings.