Highly recommended to check out the worm simulation they mention: https://openworm.org/

I wonder at what level of simulation complexity do we have to worry about any pain the simulations may be experiencing?

If you simulate a brain, assuming you do it well enough, you’ll start getting into ethical concerns.

>scientists hope to better understand how neural circuitry dictates and predicts the activity of individual neurons

Not circuitry. This is a metaphor which has passed its sell-by-date. It obscures rather than enlightens.

tangential - brain "word map" https://www.nature.com/articles/d41586-019-00069-1 (link to the interactive model there https://gallantlab.org/viewer-huth-2016/ )