I recommend rice bran or oat bran. Both are "soft" fibers and provide a lot of B vitamins. And they do not cause bloating like psyllium.
As for alkalinizing this is pretty much a myth. The body maintains its pH without assistance except in very rare instances such as some poisonings, severe COPD, uncontrolled diabetes, hyperventilation, or milk-alkali syndrome. I touched on this recently:
If people study pH regulation by the body, from medical instead of sales sources, then they would see what I mean. The body has several mechanisms to control pH to prevent the blood from going too alkaline or too acid since we cannot survive in either condition. So trying to force the pH one way or the other just puts a lot more stress on the body as it has to work even harder to get itself back to its normal pH.