career switch at mid forties

After completing your CA, you would be seen as a fresher in any niche industry.
Given your background, my suggestion would be
1. Complete your CA in the very first attempt.
2. Join a mid-size CA Firm.
3. Gain understanding of the work environment and the client businesses.
4. After 1-1.5 yrs, decide whether you want to do a job or start your own practice as now you will have CA firm experience and good client contacts.

All the best!