Perhaps you should get your priorities right first, if you want to join FSP then you need to accept the fact that the private sector pays a lot better. The fact that you are already concerned with the possibility to conduct side-business while in FSP shows that you realise that FSP pay scale won't be able to meet your requirements
If money is a major concern, then become a doctor, data scientist, actuary or an accountant.
There is a shortage of data scientists all over the world and the pay is extraordinary
I know neurosurgeons who make 50+ lacs a month at aga khan hospital here in Karachi.
No offence, but aiming at mbbs, ca and css is sheer stupidity and a waste of time. Choose a career, stick to it and work on advancement in that field.

Hope this put things into perspective!