Philosophy is intrinsically about things where there's no clear, objective and universally acclaimed answer. So it's impossible to avoid opinions, even when coming up with a reading list. Every person is going to recommend a different one. I'll give you mine, specifically for you, but I can't pretend it's objective:
First, I'd recommend symbolic logic, because it's at the nexus of mathematics and philosophy, so it would be a good point of overlap with your prior studies. It will also help you understand the structure of philosophical arguments.
Next, I'd recommend Plato, because he's the most important and central figure in Western philosophy, and nearly everyone who came after him was influenced by him. There's a lot of Plato to read, but Meno would be a good introduction (it's on the topic of mathematics). Apology and Republic are his most famous and influential works.
Next I'd do Descartes' Meditations. They are short, easy to read, and hugely influential. Plus, Descartes was heavily interested in the same question you are--what can we know for sure? (He was also a master mathematician, whose invention of the Cartesian plane revolutionized the field.)
If you start with those, it will give you a good foundation for understanding anything else you might read after that. I won't include many follow-ups because I suspect your philosophical tastes and mine don't coincide, but if you're open to reading something VERY different, I'd recommend Laozi's Dao De Jing.
All of the above should be freely available online in multiple different translations.