However, those teaching with authority-- bishops, popes, theologians, etc-- are also not giving “opinions”. They are explaining Church teaching. So, no you can’t reject those things that they wrote/taught that are actual Church doctrine. You are free to disagree with their conclusions on items that are not defined doctrine.
Of course, not everything a Saint writes is in the realm of doctrine.
Various saints write/wrote about various things and have various places within the Church. St. Thomas Aquinas was a theologian, but his writings are not considered doctrine/dogma. Like all theologians, his are speculations/discussions about theology, conclusions and proposals, but only the Magisterium decides matters of faith and morals.
It is the same for approved apparitions. They are declared “worthy of belief” by the local bishop, if they don’t contradict Church teaching and are beneficial to the faith, but they are not doctrine/dogma and so no one, Catholic or otherwise, is bound to believe in them or do whatever the seer says the heavenly personage told them to do.
If a person is “beatified” part of the process involves validating that there is nothing in their writings “contrary to faith and morals”. Every beatus with written work left behind, has therefore had that written work in a sense approved by the magisterium as “useful” for faith and moral purposes. This is not the same as infallibility. A statement need not be accurate to pass the test of not being “contrary to faith and morals”, it simply cannot have anything explicitly heretical in it.
Augustine, Aquinas and other great saints have taught erroneous and speculative beliefs that we are not required to believe. We can read their writings, nonetheless, in the assurance that there is nothing dangerous to the life of our souls in them, even though not everything contained therein will be accurate.