In case you haven't noticed, most youtube content is marketing something.

Sometimes it feels like the overt ads trivially overcome by adblockers or youtube-dl are there just to be something extremely obnoxious you can circumvent and feel relief from while still consuming marketing that is only made desirable by contrast.

They're quite easy to skip on most content. Some even put a "loading bar" to easily identify when it's going to end.

So the content creator has decided to insert their own ad inside their content resulting in a worse user experience (since you seem to be complaining about it). Should Youtube ban in-content third-party ads?

I think you mean sponsored content which is not quite the same as ads/