Arduino for STM32

I think it is a bit presumptions to talk about what others think about the board. You dont like it, dont use it. I did some testing setting it up as a simple synth with I2S output, worked great and less hassle than using I2S on most STM32 boards I have tried. Setting up the tools for C++ development...

every variant have a specific define, to see them use verbose output from compilation and look at the command line sent to the compiler, all the defines can be seen there (its a looooong line of includes and defines).

I cant se why you would use pulldown on the sensing outputs, columns, they are driven to 0 or 3.3V
make a circuit diagram and consider what happens

One resistor to ground before the multiplexer, the series diode should not be needed. The walue of this resistor should be adjusted to be approximately equal to the velostat layer resistance for one crossing when half depressed. If you dont have a pull down resistor the the sense line becomes "...

Strange , I would have thought the pulldown resistors should be placed on the line inputs to the sensing multiplexors. Thus crating a voltage divider:

Column 3V - velostat rsistance - Out put Row sensor line to ADC - row resistor - ground.

I would say C/C++ as such is not much larger footprint than assembly code, the compilers are more efficient the anyone but expert assembly gurus. The differences in size is the added code libraries from the core, mathlibs are a fair chunk, gpio and communications, that will make further use of these...

No one here can reproduce your issue, and you dont even show what elements of the original has changed, so help is very limited.

And yes the web server can send lots of data, but each frame is limited.

Program seems to be overflowing serial buffers, looking at the timestamps,

the Serial2.println(elapsed) seems to get called ok,
and so should signalWatchdog(), but that function is empty so how do you know its not called ?

Does the sound work ? The time display discrepancies are most likely because the SAI_Receive inside the timer irq blocks the SysTick interrupts and therefore makes the micros() nonfunctional. Also note that variables used to communicate between IRQ handlers and loop level should be marked as volatil...

SPI ??