Splitting the ciphertext with more slots into a number of ciphertexts with fewer slots was described in the answer to your previous question. However, all ciphertexts have the same ring dimension and are encrypted under the same key. In order to make the ring dimension smaller, you need to perform field switching, described in https://eprint.iacr.org/2012/240.pdf, which is not currently available in OpenFHE and you would have to implemented on your own.