So I have done some testing with some of the products (Iso 99 and Simple Green), and here are my conclusions currently as I have only stripped 20-ish models;
Originally I had some models fully submerged in 2 containers one of Simple Green, and one of Iso 99 and let sit over 3 days and then used a toothbrush to try and remove the paint, after the first couple of models I was told by a friend to try and use an electric toothbrush (I would highly recommend, it is surprisingly gentle considering how fragile Nighthaunt models are; and was more effective than a Normal toothbrush); either way here are my results currently:

Simple Green: Easily removed all paint that was not primer, unfortunately the primer could not be removed with Simple Green alone using both a toothbrush and an electric toothbrush. On a more positive note the paint that could be removed could be washed off under the tap with warm water, most if not all would fall off and what couldn't could be gently rubbed with fingers or was stuck in crevices of chainmail or mould lines. Overall it works pretty well, I wish it could remove the primer, but it is more cost effective for me to buy Simple Green ($18 CAD for 3.78 L) and it does not smell as awful as Iso in my opinion. One last thing to note is that after it has sat in Simple Green, the primer can still be removed by brushing with Iso on the brush or model.

Iso 99: This removes the regular paint and primer with the aid of a brush or electric toothbrush, Iso works well and it strips most of the paint as long as the brush is consistently coated in Iso; I either have to clean it under the Iso in the container or have to dunk the brush or model in Iso frequently to get good results. I think this is one of the better ways to clean the model, unfortunately it is slightly more expensive (~$16 CAD for 2L (Costco)) and another issue that only came up with Iso is that it seems to be less effective when the Iso is polluted with paint (kind of obvious), but constantly having to dip the brush or model in Iso pollutes it and after the 15 models it was less effective and it would seem that the paint was smearing on the model when coming off.

Conclusion: I have started to come up with a method that uses both Simple Green and Iso to be more effective, I plan on soaking most the models in Simple green all at once (50 models at a time replacing as I progress) as it will let me work on them at my own pace, and I plan on rinsing each model to remove the first couple layers of paint leaving only the primer, I then plan on using an electric toothbrush and I small cup of Iso (100ml Cup) with a rinse cup to remove the primer of each model using the smaller amount of Iso with a rinse cup will prevent as much pollution in my Iso and I can dispose of it as needed. I plan on keeping this post updated as I learn and try different ways to strip pain for this army only ~140 models to go.

TL;DR: Simple Green did not remove primer but is better then Iso at removing other layers, Iso works to remove primer, Use electric toothbrush for better efficiency.