The even length substrings which equal their own reverse complement are precisely the even length palindromes when you reverse every other letter in $x$. Therefore you are looking for the number of strings of length $n$ that do not contain any palindromes of length $2\ell := 2\lceil k / 2 \rceil$. Let's call this number $f_\ell(n)$.
Counting the number of words of length $n$ avoiding length $k$ palindromes sounds like a very natural problem in combinatorics on words, but I couldn't find anything after a quick google search, so I'm not sure there exists a nice exact formula. However you can find some rough bounds on its behavior for variable $\ell$ and $n$ using counting and probabilistic arguments.
For example, there is an upper bound of $f_\ell(n) \leq (2^{2\ell} - 2^\ell)^{\lceil n / 2\ell\rceil} \approx e^{-(2^{-\ell-1}/\ell) \cdot n}\cdot 2^n$, because there are $2^{2\ell} - 2^\ell$ possibilities for each segment of length $2\ell$.
For a lower bound, note that cyclic strings of length $2^{\ell-1}$ are expected to have half a palindrome. Thus there are at least $$\frac{2^{2^{\ell-1}}}{2\cdot2^{4\ell}}$$ different strings of length $2^{\ell-1}$ same length $2\ell$ prefix and suffix, which we can concatenate arbitrarily without introducing palindromes. This gives a lower bound of $$f_\ell(n) \geq \left(\frac{2^{2^{\ell-1}}}{2\cdot2^{4\ell}}\right)^{\lfloor n / 2^{\ell-1}\rfloor } \approx 2^{-(4\ell+1)2^{-\ell+1}\cdot n}\cdot 2^n.$$ Therefore $f_\ell(n) = 2^{n(1-2^{-\ell + o(\ell)})+O(2^\ell)}$.