Depends what you want long term.
If you have your heart set on an airline job at any point in the future, you would need to do ATPL groundschool.
Once you have passed all 14 exams, you have 3 years to get both your CPL and an IR, if you don't manage this then the groundschool will expire.
So you can get your CPL/FI rating, and start instructing, but you need to keep an eye on that clock for the IR or you lose the groundschool, and then wouldn't be able to apply to any airlines anyway.
With the IR being the most expensive bit, and only valid for a year, it makes sense to leave it until last, and when you are fairly sure an airline interview will come along.