Former investigator, I left the investigations world years ago but am still working in the security-cleared world.
I agree with the folks here saying that the best way to get the training to enter this field (and the required security clearance) is to get an offer as a full employee (not 1099) with one of the companies performing investigative work for the government.
In terms of quality of life in this field, everyone has their own opinions, and each person’s situation is unique so it’s disingenuous to paint with broad strokes and say this is a good or bad field for everyone. And even then, the quality of your team lead matters a LOT as well.
I will say that some of the biggest complainers about this field also put the least amount of effort into actually doing things to help get out of this field (re-skilling, taking courses, getting certifications, etc.), so while a LOT of the negativity associated with this career can be justifiable, take it with a grain of salt.
This field got me in the door to the national security world and I still use a LOT of what I learned as an investigator in my current role, so while I would never go back to this field, I do look back on it fondly as a crucial stepping stone (I never had any intention of staying in this line of work long term though, so your mileage may vary).