Rockets don't go a max of 120,000 ft. It's extremely hard to send rockets even 10,000 feet. But 120,000 is something else. At that point it barely even qualifies as "amateur." People can and have gone higher, but the highest REAL amateur rocket has gone to 73.07 miles in 2014:
https://en.wikipedia.org/wiki/Civilian_Space_eXploration_Team#Space_Shot_2014_"GoFast"_Rocket, or 385,810 feet, more than 3 times higher than just 120K feet. Before that, the same team sent one rocket 72 miles in 2004. They have a history of doing that, though they are HUGE projects.
Videos don't show the horizon to eye level. It'd be really difficult to even measure this, since the rocket spins, turns, etc. This means the horizon is constantly moving throughout the frame. A better way to determine this is a weather balloon. Much more stable, though it will not go nearly as high. In these videos you can see the horizon drop.