I followed the sources they link and the ones those link and found that the best substantiation they have is "according to the accounts collected by some NGOs".

Here's some more up-to-date info regarding a previous ICJ case against Russia, so we can weigh up how much these accusations might be based on fact. Spoiler alert: every accusation is a confession. I'll quote a relevant bit to your atrocity propaganda peddling:

The Court has held that certain materials, such as press articles and extracts from publications, are regarded ‘not as evidence capable of proving facts.’

And this one, regarding the reliability of testimonies presented by Ukraine.

The ICJ was also highly condemnatory of the quality of witnesses and witness evidence produced by Kiev to support these charges[...] Statements attesting to this were “collected many years after the relevant events” and “not supported by corroborating documentation”

I don't doubt there must be unspeakable shit happening, there's been a war for so long that monsters are bound to take part. But I'll hold my judgement as to how systematic it is until evidence is presented (which it absolutely hasn't been), not just claims by notorious liars who said the same shit about Hamas without any evidence and no pushback from these very same publications.