Please do understand, that I actually believe that your intentions are honorable, that they are wrong is another issue

I am not an embedded expert
so why do you do these things, should they not be left to "an embedded expert", what is the likelyhood that someone stating "I am not an embedded expert" will end up with solutions that take into consideration the uniqueness of small embedded

I guess it's much easyer to use a function SPIWRITE instead to configure 20 registers before sending one byte.
a) that is NOT 'abstraction' that is a module
b) how do you propopse to do this in an universal way, should all calls include which pins the SPI is connected to, which processor it is (HW SPI or not - and which type), which ..... Of course, you can handle that in "SPIinit()", but either way will leave your 'abstraction' with so many code switches that it more likely will be an 'abberation'. If you intend to dictate e.g. which pins SPI will be on, I predict that your 'abstraction', in the best case MAY get ONE user besides you.

Such features are abundantly provided already NOT as 'abstractions' but as freely available source (or object, if you prefer) modules.

Erik