1) There is no way to monitor/prevent abuse for kid to get exposed to extreme (literally, potentially mentally damaging) pornography and perversion that percolates into the mainstream life via innocent channels - manga, anime, Reddit, and so on. When I grew up the content of that sort was significantly more limited, i.e. occasional porn GIFs, most fine erotic material, but nothing of the extreme sort you could easily stumble upon by just being online today, etc).
2) Qustodio can partially filter out some content as it's installed on the PC and the smartphone, but due to the all-pervading encryption it has no visibility into Discord and other social networks.
3) Disastrous breaches in IT school policies. School has given mandatory Chromebooks to all kids which they use probably half of the time in the class and for homework, and there is no way to bind the school account to Google Family Links - this is forbidden by the school admin. There is some filtering on the school Wi-Fi which school kids circumvent by joining their phone's shared Wi-Fi. Complained, received boilerplate "we are doing our best" reply. This is an independent school in London.
4) The only option I have is to begin breaking SSL in the middle via, say, FortiGate and hope it will catch some of the sites. This would also be applicable only to home Wi-Fi, rather ineffective.
As a parent, the only solution I have is to prohibit access to screen, i.e. take all devices away, completely. Which I have failed, as there are very little communal spaces for the kids to hang out and interact where I live, and those who do hang out tend to be vaping, drinking alcohol, etc. All the kids are online in the evenings - chatting, sharing memes, laughing together, or they have pre-arranged playdates.
So, my choice here is to completely take away all devices and, hence, completely remove the social component. Or just pray he won't get abused online. I, honestly, don't know what to do. I have been living with computers since 80s, started out with ZX Spectrum and PC/XT, and I am horrified by what allowing the kid to roam Internet without filters may lead to now.