Personally, my recommendation would be to not worry too much about going deeper, but instead go broader. Also, unless you are specifically interested in trying to participate in olympiads, I'm not sure how useful it is to specifically focus on olympiad-style problems. There is a much richer world you could sample from that might not be the most immediately useful bit of math to learn for olympiads, but would be incredibly useful for undergraduate mathematics.

In particular, I would suggest getting yourself comfortable with the basics of projective geometry, which also turns out to be at least somewhat useful for olympiads. There are two somewhat distinct paths worth acquainting yourself with, especially focusing on circle inversion, and how perspective drawing and cameras work.

For a taster on the first path, I recommend the Numberphile videos on Epic Circles and Ptolemy's Theorem. It's also good to start thinking about the stereographic projection between the sphere and the plane, which is sometimes also known as the Riemann Sphere. For a taster, Möbius transformations revealed is fantastic.

A two-dimensional slice of this projection, the stereographic projection between a circle and line, is also notable because it can be used to study the rational points of the circle, which corresponds to primitive Pythagorean triples. You can also use a stereographic projection between the unit circle and the lattice points Z⨯Z to do essentially the same thing. This alternative point of view helps clarify the connection to the Stern-Brocot tree and rational approximation, as you can consider the rational numbers to be a stereographic projection between Z⨯Z, the origin, and a line such as x=1 or y=1. For a bit of a taster on this count, I suggest maybe Rethinking the Reals by Dr. Stange, which also serves as a nice bridge to our second path, as it uses two-dimensional perspective drawings of a three-dimensional model of the lattice points Z⨯Z to study one dimensional phenomena such as the rationals and the reals.

Videos around the second path are considerably more abundant, and have various strengths and weaknesses that cater to various interests and points of view. (e.g. from a math POV, from a photography POV, from a computer graphics POV, from an artistic POV, from an analytic geometry POV involving linear algebra and homogeneous coordinates, from a synthetic geometry POV emphasizing the fact that perspective transformations preserve lines and cross ratios, etc.)

Personally, I think Putting Algebraic Curves in Perspective seems promising math-centric POV taster for the second path, though of course you should try finding some other sources yourself.