One of my plans for my retirement days is to make a cubesat that uses a solar sail to gain momentum and then somehow give that momentum to space junk so that the junk falls down to atmosphere faster. Then use the sails again to increase the momentum of the space garbage collector for the next piece of garbage. Unfortunately this plan has still quite a few gaping holes in it.

No not really, the massive stuff yes..maybe, but small stuff no. Just imagine the power you need to go on the same orbit and around the same speed just to pickup a fist sized trash piece, then you need energy again to slow it down so it can fall back to earth, or you take i with you (and that needs more energy too for you next trash pickup, because your mass got bigger).

It's the old story: Oh we cut all the trees, time to invent a machine that can extract CO2 from the atmosphere.

Elsa-D is up there now running some tests.

https://asia.nikkei.com/Business/Aerospace-Defense/Japan-s-A...

3 major options are discussed in https://youtu.be/yS1ibDImAYU

Edit: that's an 8 minutes Kurzgesagt video.

You can shoot lasers at it and the vapor knocks it out of orbit.