Arduino for STM32

Provide the board details, stm32 chip used etc.
there are a large number of stm32 soc all with different features, capabilities.
board details is also important, if it is well known board, a board on the list
https://github.com/stm32duino/Arduino_Core_STM32
mention it, otherwise provide more details ...

fpiSTM is correct

it is a bit surprising that they put a usb-uart (ch340n) at pa9 / pa10 which is in turn connected to usb, as normally stm32f103 has its own usb.
stm32f103mini_sch.jpg
in this case, you'd need to use the normal uart option for Serial in your sketch

and it use buttons instead ...

To program stm32f103c8t6 blue pill via uart you need to set boot0
and you need stm32cubeprogrammer (or some such programming util)
https://www.st.com/en/development-tools/stm32cubeprog.html
^^ this is from ST (you can consider that an official programming tool)

and a google search leads to various ...

it involves messing with ldscripts and codes to place codes / functions in different addresses / sections

https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_21.html
https://www.rkoucha.fr/tech_corner/gcc_sections.html

so that you have a flash memory layout like

0x8000000 Sector 0 16 kb ...

To build with a com port, select from the menu Tools > USB support > CDC generic supersede U(S)ART.
Then build the sketch.
note that normally the FPU is 32 bits and in usual cases at most 2 x 32 bits FP ops in parallel. This is normally handled by -O2 or -O3.
I'm not too sure if in ESP32 the ...

if you tried my sketch in the zip file, it blinks the led when you run the whetstone benchmark. did it blink?
and you should see the whetstone benchmark on the serial console when it runs

oh and that sketch can't be 474,784 bytes, the bin file bundled in my zip file is only 44.9 kB.
you only need ...

oops I'm off-topic in the prior comment
but that apparently the HSE crystal frequency is apparently not indicated on the schematic itself
https://github.com/makerbase-mks/MKS-Monster8/blob/main/hardware/MKS%20Monster8%20V2.0_003/MKS%20Monster8%20V2.0_003%20SCH.pdf

using the 'Generic' variant ...

Welcome,
I've not actually build Marlin from source, but that it is a complex set of codes given the large number of features it support particularly for 3d printing and the large number of platforms it support.
If you specifically want to use Marlin, Marlin web
https://marlinfw.org/
and its ...

well install it the usual way first
https://github.com/stm32duino/Arduino_Core_STM32/wiki/Getting-Started

then find it in the core install folder
Windows: C:\Users\{username}\AppData\Local\Arduino15.
macOS: /Users/{username}/Library/Arduino15.
Linux: /home/{username}/.arduino15.

and copy your ...

it is possible to write directly to flash instead of using EEPROM emulation, these are mostly covered in the FLASH sections of the ref manuals.
The way flash works is that if a page is erased it is reset to 0xff for all bytes, writing data to them toggle the bits to 0.
Hence, it may be possible to ...