I really don’t think classes helps much, to me biggest help were SC-1000 this grammar books and
OG explanations
In month plus time frame if you can really spend 3 to 4 hours a day and can spend some time on weekends to study more grammar you can really achieve good results.
In SC I improved from hit rate of less than 50% to around 70%, most of the improvement is due to pattern recognizection, after doing lot of SC (around 500 of them) I can see some patterns, for eg. in idioms I am quite comfortable with not only but also, as good as or better than, consider XY etc, in S-V agreement I can really pick complex subjects etc.
Other reason for improvement is just becoming more comfortable with grammar as such, now if I read
OG explanation that this is verbal or gerund I know what they mean which helps creating kind of road map in mind what they are up to.
The overall trick is to find out your strengths and weaknesses and zoom in to them, I now strongly believe that with proper preparation you will at least solve 50% of hard SC. And off course I believe anyone who can score well in Math can score well in CR.
Now once you are good at SC and CR it will reduce some time pressure on RC and overall performance will improve drastically!
This is overall philosophy I am relying upon; to me, it seems logically correct I hope it will practically prove valuable.