Solution: This was user error, solution here.
I am reading data from an ADXL355 accelerometer via SPI on a RPI Zero W and seem to have gotten stable g values after conversion, but the data I am getting seem to have a few problems.
Link to datasheet: www.analog.com/.../adxl354_355.pdf
1) Offset from expected value
Even though the accelerometer is sat on a static table, the data from it do not read 1g. There are settings for changing the offset of readings, but it strikes me as strange that the offset would be set to several g's, around 3g in a +-8g range. This offset is present in the x-axis and y-axis (the z-axis seems to be way out of order), these are shown in the following image.
2) Axes movement in same direction
I tested the accelerometer axes by changing the orientation of the board as per the following image.
This gives the following readout from the accelerometer:
The y-axis being displaced in the same direction at each extreme of the rotation is what surprises me, given that the endpoints of the rotation are 180 degrees opposed.
3) Coupling of axes
As seen in the accelerometer testing results image above, the axes seem to move together, this is something else that confuses me, as I am barely moving the x axis. Could it be that it is being rotated through space as though it was attached to an arm?
4) Z-Axis Irregularities
Any ideas as to why the z-axis is all over the place would also be greatly appreciated. The axis are all fed through the same conversion from the data read directly from the accelerometer to give g values.
Added solution note
[edited by: ComteCristo at 12:34 PM (GMT -4) on 1 Aug 2022]