I bought the StarTech dual 4K DisplayLink adapater after watching one of those videos. I use it to power a second 4K monitor with my M1 MacBook Air running lid-closed. I have a 1080p gaming monitor as my primary connected to a Elgato dock via DisplayPort.

DisplayLink is definitely suboptimial, but it's the only option. There is noticable input lag (my guess is around 100ms) that makes mouse and keyboard input feel sluggish. Showing a moving image on the screen pegs the DisplayLink Manager process to 100% of one core while it's playing. DisplayLink does not currently support rotation on Big Sur, though it probably will in the future. However, the image itself is pixel-perfect, so it's fine for secondary apps like chat or monitoring.