As I have said before I know two instructors without CPL TK and they both work full time as FI's. They fly and earn just as much as their colleges who have the CPL TK.
The only restriction is that they can't teach ab initio PPL students. But they can teach for the LAPL, NPPL, SSEA, SLMG, Microlight, SEP, TMG, Night, IMC and aero's. They can also do all forms of differences training, one hour dual with the instructor, check out flights and trial flights.
I'd suggest do the FI rating first then if the work doesn't come do the CPL TK.