dcrtBits represents the size of the scaling factor, which is directly related to precision. firstMod is represents the number of bits of the modulus on which the decryption happens (assuming you consume all levels) which also affects the precision (rather correctness) of the returned result. Precision is also affected by using modes such as FLEXIBLEAUTOEXT, which incurs the least error at the cost of an extra small prime. To see concrete relations between the error (therefore the precision) and the ring dimension, please read Approximate Homomorphic Encryption with Reduced Approximation Error. For some higher-level useful discussions related to setting dcrtBits and firstMod in CKKS, see Meaning of the bit precision Bis, How should the outputs of the Precision Estimator be understood?, Is there a way to estimate the precision for a calculation?, Setting security parameters according to 128 bit security (very small CKKS scaling factor),
That being said, a ring dimension of 2^9 is very small, and 128-bit security might not be achieved with it (even for additive FHE, so multiplicative depth 0). You can consult the tables in https://eprint.iacr.org/2024/463.pdf to get some sense of what size of ring dimension would correspond to 128-bits for certain ciphertext modulus.