Problem solved
¡Gracias Ron!
Problem solved... with the KISS principle still intact.
I implemented a single-pole RC filter for the upper end with:
R = 39 K
C = 0,0033 uF (permanent)
Later, I programmed the micro in charge to switch, via NPN transistors, additional caps at two convenient frequencies, in parallel with the "permanent" one.
At 307 Hz C = 0,01 uF (added in parallel to the above)
At 36 Hz, C = 0,1 uF (added in parallel to both above).
Today is a nice day! A saga of false starts, wrong designs, lack of adequate testing and ignorance of basic concepts, lasting several years, is over.