I want to second the idea of having granular scales for each category of material.
It would be nice to have a distinction between games like Doom Eternal and walking simulators like Firewatch (both are M-rated). And has been stated before, rating systems are inconsistent across different countries, and people have different preferences on what type of content they are okay with.
I would like to see a system that has 4 levels (e.g. none, suggested, mild, explicit) for the following categories: violence, nudity, sexual content, language, drug use, and maybe a category for psychological distress (i.e. horror). That would cover the whole gamut of content. It would make the rating system more culturally agnostic. It would be easy to filter in search results. It would also future-proof the rating system because you can programmatically assign E, T, M, and AO ratings. From a UX perspective, it would also force content creators to think about the content so that works are not inappropriately assigned, which happens all the time on sites where the NSFW flag is opt-in.
I understand the merits of a simple rating system; it's fast. But the gap between "viewer discretion advised" and "Adult Only" is too large. A user setting their rating restrictions is something that people generally only have to do once, or very infrequently (i.e. searching for a friend or family member), so I wouldn't worry about it being too difficult. Just include it as part of the user onboarding process: 4 clicks, and done.