How to compute the attacker's probability?
The probability of recovering the string $R$ is the same with both of your scheme. The reason is that your second scheme can be separated into two independent parts. In the first part, you just transform $R$ into $R'$ using only your step 1 (this is invertible and from $R'$ you can recover $R$).

In the second part, you just encrypt $R'$ with the previous scheme. As a consequence, the probability of recovering $R'$ is the same as in the first scheme and due to the invertibility of the first part, this is equivalent to recovering $R$.

I am wondering about why you want to use this scheme. Did you consider (in your context) attacks that would approximate $R$ instead of predicting it exactly ? I ask this because this would greatly increase the probability of success.