I too think the 2400G looks like the sweet spot. The Ryzen APU's are based on the previous generation, so 2400G is like a 1000 series Ryzen CPU and the 3400G is like a 2000 series Ryzen CPU. The advancements brought to the new 3000 series CPU's won't hit the APU's until they release 4000 series APU's next year (hopefully with Navi graphics cores).
Hahaha, I was just about to link you to the same video that you already linked. It has video playback tests starting at 8:35. Seems pretty definitive to me. If you have playback issues like screen tearing then there is a problem with the drivers or settings. The hardware is pretty capable of all basic tasks.
In that video, he has two separate builds listed, and you got the parts mixed up. The 500W PSU doesn't go with that case, as it has no space for anything other than a PicoPSU. He uses cheap knockoff PicoPSU's in a lot of his builds. I have one from Mini-Box that is 125W with an appropriate power brick and it is really at the absolute limit of what it can do. Under full CPU and GPU load it can draw in excess of 110 watts which virtually any ATX PSU can provide, but you want a bit more than the ~10% headroom I have if using a PicoPSU. Especially with the cheaper ones, you don't want to push them that hard.
I would strongly recommend avoiding itx and ultra small cases. There are very small MicroATX cases that can fit standardized components like ATX PSU's with far less hassle, better upgradeability, and best of all cheaper than paying that big tax to go just a little bit smaller. And that's coming from a huge tiny house fan; I've never seen a house that physically didn't have the space to cram a MicroATX PC inside. I see lots of threads from people with fitment, heat, and other issues on extra small builds that are rarely an issue by going the next size up. Keep that in mind before throwing down your money.
If you insist on going small, then the ASRock DeskMini A300 may be the way to go: