Philosophical question about Pi and connections in maths

There are two common ways of defining $π$, and they are interestingly and non-trivially equivalent. $ \def\lfrac#1#2{{\large\frac{#1}{#2}}} $

If you wish to define $π$ as the ratio of a circle's circumference to diameter, you would first have to define arc length, which is highly non-trivial. You also need to prove that every circle has the same ratio, otherwise your choice of definition is ill-founded. After you do that properly, you can then take a point that traces the unit circle from $(1,0)$ anti-clockwise at speed $1$, and then define $(\cos(t),\sin(t))$ (for real $t$) as the coordinates of that point at time $t$ (equivalently after it has traced an arc of length $t$). Then you can prove that $\lfrac{\sin(t)}{t} \to 1$ as $t \to 0$. Many textbooks give a 'proof' that squeezes between areas, but this is invalid unless you also have proven (or assumed without proof) that the area of the unit circle is $π$. No, it is not obvious and not easy without some calculus. But it should be now clear why the limit is completely determined by your choice of definition of $π$. If you go this route, you also will have trouble defining the complex-valued trigonometric functions later in a natural and intuitive way, which is why I prefer the other common approach below.

The main alternative is to define $\exp,\cos,\sin$ expressly to solve useful differential equations. $\exp$ is the unique complex-valued function such that $\exp' = \exp$ and $\exp(0) = 1$. The motivation is that using such a function $\exp$ we can solve $\lfrac{dy}{dx}+f(x)y = g(x)$ for variables $x,y$ given functions $f,g$, by considering $\lfrac{d}{dx}\Big(y·\exp(\int f(x)\ dx)\Big)$. Also, simple harmonic motion requires solving $f'' = f$ for a real function $f$. By considering the desired functions' behaviour around $0$, we can discover them intuitively by using a polynomial approximation as described here. It is non-trivial to prove that taking the limit (the infinite series) produces the functions we want, and the linked post shows an elementary proof.

The same intuition suggests the series expansions for $\cos,\sin$ as well, and we can observe that we can define $\cos,\sin$ via $\cos(z) = \lfrac12(\exp(iz)+\exp(-iz))$ and $\sin(z) = \lfrac1{2i}(\exp(iz)-\exp(-iz))$ to get the desired expansions, and then can easily prove all their properties based on the properties of $\exp$. For example $\sin' = \cos$ and $\cos' = -\sin$ (by the chain rule on the above definition). It is a matter of preference whether one proves general facts of power series to get termwise differentiation, but here I am describing a bare-hands elementary approach.

I shall sketch how to get all the basic properties of $\exp$. From $\lfrac{d}{dz}(\exp(z)·\exp(-z)) = 0$ we get $\exp(z) \exp(-z) = \exp(0)·\exp(-0) = 1$ (by a suitable application of Rolle's theorem). Similarly $\lfrac{d}{dz}(\exp(z+w)·\exp(-z)) = 0$ for any constant $w$, and hence $\exp(z+w) = \exp(z)·\exp(w)$. Thus $1 = \exp(0) = \exp(it)·\exp(-it) = (\cos(t)+i\sin(t))·(\cos(t)-i\sin(t)) = \cos(t)^2+\sin(t)^2$. Now for every real $t$ we can see from the infinite series that $\cos(t),\sin(t)$ are also real and hence are the $x,y$-coordinates of $\exp(it)$. Thus $|\exp(it)| = 1$. Next we have $\left|\lfrac{d}{dt}(\exp(it))\right| = |i\exp(it)| = 1$ and so $\exp(it)$ travels along the unit circle at constant unit speed.

At this point, if you wish you can use arc length to define $π$ as in the first method (with its caveats) and get $\exp(i2π) = 1$, but I shall show a purely analytic alternative approach that literally follows the intuitive idea that $p = \exp(it)$ traces the unit circle as the real parameter $t$ increases. Note that $p$ cannot stay within the top-right quadrant (namely $Re(p) > 0$ and $Im(p) > 0$), otherwise by the extreme value theorem it would reach some left-most position but at that point we would have $Re(\lfrac{dp}{dt}) = Re(ip) = -Im(p) < 0$ and so it must continue moving left-ward (by Rolle's theorem again). Since $Im(\lfrac{dp}{dt}) = Im(ip) = Re(p) > 0$ in that quadrant, $p$ must first exit the quadrant by crossing the imaginary axis rather than the real axis. That crossing point must be $i$, and we can define $u$ to be the smallest positive root of $\cos$ so that $i = \exp(iu)$. Then $\exp(i4u) = i^4 = 1$, and therefore $\exp$ has period $4ui$. One is free to define $π = 2u$.

Whichever property you use to define $π$, it takes non-trivial work to prove the properties that you did not use to define it. So there is no circularity really, and the elegance of the relation between the exponential function and trigonometric functions actually has quite complex underpinnings, which makes it marvelous!

Also, there could have been a misconception in your question of why an infinite sum of rationals can reach $π$. The fact is that every real number can be reached by some infinite sum of rationals. The decimal representation is one common instance, where each real number is represented by some infinite sum $n+\sum_{k=0}^\infty a_k 10^{-k}$ where $n$ is an integer and $a_0,a_1,\cdots$ are integers in the range $[0..9]$. It is only because $π$ is special that we are interested in all sorts of infinite series that sum to it. The one you mention is an especially curious instance. If you want only infinite sums of rationals following some algorithmic pattern, then what you get is exactly the computable reals. Nearly all mathematical constants that mathematicians have defined are in fact computable, but some like Chaitin's constant are not.

Concerning the limit$$\displaystyle\lim_{x\to0}\frac{\sin x}x=1,$$you stated that it “is true if and only if $x$ is measured in radians”. This is false. The sine function is a function from the reals into the reals. So, when, in the context of this limit, someone speaks of $\sin x$, then $x$ is a number, not an angle, and thefore asserting that “$x$ is measured in radians” makes no sense.

Of course, originally the sine was the sine of an angle, not the sine of a number. But we are dealing with the limit $\lim_{x\to0}\frac{\sin x}x$ here and, in this context, the sine has always meant a function from the real numbers into the real numbers. That's how Euler dealt with this limit in his Introductio in analysin infinitorum, published in 1748. I suggest that you read the short article Why the sine has a simple derivative, by V. Frederick Rickey.