I think my method will be less conistant - and especially more risky - than yours in the end, but faster for TAS situation (only one jump required).

I'm trying to find a good setup for TAS but I keep falling to my death lmao (I got it working a few times but too slow)

Instead of jumping before the kick, you can also charge the kick. After charging it a tiny bit, Billy will "fall", but if you release B withing 2 frames of falling, he will go through. I've been dying a lot with this though.

I have no idea why this works, but it looks really nice. Good job !

For Sand Ruins, you not only need for the bee to get you up there, but you also need your egg to come, because there's no egg later. Also, clipping in the sand is insanely hard for some reason, we would need to find a consistent way to do it.

You can clip pretty much anywhere around the pyramid, and even on some other platforms (see 6-3 in this tas at 54:28). It's pretty random though, which makes it really hard for RTA.

For RTA, the best thing would be to do the parcour at 48:45, but then go to the left instead of going to the bees. The boss door is right under that platform, but Billy can't seem to be able to clip in this room for some reason.

Using the bees like in the TAS is impossible in real time. Getting that skip would allow skipping a long cutscene and some slow hoops.

It is indeed this platform, you can access it in 6-2 like this : (I believe easier ways were found to go there now).

When you kick the egg, it will try to come back to billy no matter what, so clipping in the sand after kicking it will make the egg push Billy on the side, and the plan was to get billy inside the Boss Door room on 6-2. I managed to get the egg go there easily, but Billy can't go through the wall it seems.