Some good advice there however there are other points to consider.
If you only want to be an instructor to build hours and stay current; will a school want to employ you? Whilst most schools are used to hour builders they have a business to run, and may be reluctant to employ an instructor who can't deliver the goods when they want them.
It could take you 18 months to two years to find an airline job, you could qualify as a FI and take 6 - 12 months to find an instructing job!
A number of FIs with 1500 hous plus, have not got airline jobs whilst Instructors with 600 hours have jumpped the queue; something to do with amortising the cost of training versus the probability of you moving on.
Stick with the day job; do a FI course if you can, and instruct part time; that way you can afford to pay for it and stay current. Leave the full time jobs for the old boys on pensions who can afford to be FIs.