I'm only half joking about this, but you wrote a really long post with hypothetical scenarios and imagined backstories about a video maker you don't seem to know personally. And then went on a diatribe about "art schooling", which you seem to have not personally experienced. All that text to...I guess ask if and how we would tell someone if their skills suck? I wouldn't have pointed this out in any other post, but since the title is what it is...I'm wondering how to break it to you that you should be more succinct in the point of your discussion
On to the topic of discussion - If they ask me directly, then I will be honest about it, and provide my advice on how I think it can be improved, or if it's in a skill I'm not well-versed in, provide my critique clearly on specific parts that I think looks/sounds bad to me. Maybe it's because I've attended many art critique sessions where it's always made a point that a good critique is one that is specific and clear, highlighting parts of the art piece, and I personally do not find this critique exercise very hard. And I've used this principle in every critique I give out in both art and non-art related discord servers without having experienced any big grief or drama from the artist.
There's no "agonizing" on my part either as I just use the base minimum politeness of no insults or mocking. I guess the most I do to try to avoid hurt feelings is that I try to find some part of the piece that I like and also point it out to them as a compliment, but this isn't really me trying to avoid hurt feelings so much as I feel it is also important to point out the good parts as much as the bad parts. I think it gives them "guidance" on what their strengths are and can build upon them. If they are not aware of their current strengths, there is a good possibility that they might end up leaving it behind.
Obviously, if the critiques handed out are just a very vague "this looks bad", then no matter how polite you deliver that sentiment, it is not productive at all, dare I say it's a waste of time for you and the person asking for the critique. On the other hand, if the critique is clear and specific, then the base minimum amount of politeness is all that's needed.