I suggest adding the function of turning on the autocrystal in the settings.
The function includes the ability to click with an empty hand(right click) on the obsidian will automatically switch and put the crystal on it.

Not sure I fully understand, but this is possible now.

In the interaction config, add minecraft!obsidian = <id of crystal>.

The idea was that when I put obsidian and pressed the right mouse button, the crystal itself would be put on obsidian

If you mean to place obsidian and have crystals be placed on top automatically, that is out of scope for AS.

AS could switch to you holding the crystal when right clicking obsidian such that you place it, see my previous comment.