ACL Reconstruction depends on multiple factors.
Given that you have RA, I'm assuming that the knee involvement is not severe and therefore your doctor has suggested surgery.
1. You need to think about why you would delay surgery.
2. Whether youre opting for surgery immediately or waiting for a while, you will need to do a LOT of exercises to keep your thigh muscles strong. This is definitely a priority. Before surgery this is called Pre-hab.
3. Depending on how you injured your ACL you need to work on general activity also. If it was a trivial injury then it means your inherent muscle strength may be low.
Next Steps
1. do exercises whether you're opting for surgery now or later
2. fitness
3. speak to your Orthopedic Surgeon or rheumatologist for treatment of RA