Well, the SMPS you used are crap and although they say it is a LM25xx in fact they are just bad copies. Also the caps and inductors are crap too.

I would design my own SMPS and make sure to follow the manufacturer advices (either TI or Microchips). Having a goal of 20mV p-p is totally acceptable and doable.

Linear regulators will get too hot and while DC output, if well designed, may be clear enough, it is wasting too much power. You will need a big (and expensive) heat sink also, with ventilation holes on the final product.

Ioannis