Normally very little taught about crypto in undergraduate. AS long as you have the passion you are welcome to the subject.

Im just worried whether my mathematics background is going to be a large disadvantage in further studies, and whether there was any benefit of studying comp sci to cryptography. I'm currently doing my thesis on Signal, which is beneficial i guess.

it depends on the type of cryptography. There are very math heavy parts, and very applied parts. If you're interested in the math heavy parts, not having the math background will be a disadvantage, but you could take math classes in grad school

for "standard" cryptography you need some group theory at the undergraduate level, some basic number theory, and mostly comfortability with a certain set of probabilistic bounds which show up all of the time (so some basic probability theory, but mostly familiarity with things like chernoff bounds)

this is assuming you have algorithms/basic complexity theory background. If not, you'd probably need those, but again nothing at a super advanced level.

Well, the majority of researchers I personally know in theoretical crypto come from a CS background. It probably differs a bit between countries.

You'll likely lose steam by the time you attain all background knowledge. So start by diving straight into the rabbit hole. Like if Zero knowledge proofs interest you, go checkout some grad level books, or winter school lectures on youtube. If you think you lack knowledge on something, then plug that in. This is one way to deal with things.