The following is my personal view of Buddhism and psychedelic use:

The 5th precept in Buddhism is to abstain from alcohol because it causes heedlessness. By extension, other drugs such as cocaine or morphine would also be banned. But if, for example, a person is severely injured, morphine would be permissible to use as a medicine against pain.

Psychedelics, if they are used medicinally, do not conflict with the precept. If they are not used medicinally, they do come in conflict with the precept. Not coming into conflict with the precept does not mean, however, that psychedelic use is a supplement to Buddhist practice, any more than penicillin is a supplement to Buddhist practice.

In order to be a supplement to Buddhist practice, and not just a general medicine which is compatible with the 5th precept, psychedelic use must assist one in somehow recognizing, managing, transcending, and/or dissolving the three poisons of greed, hate, and/or delusion. If psychedelic use can accomplish this, it is more than just a general medicine which is compatible with the 5th precept; it is then a genuine supplement to Buddhist practice.

Conflict with the precept, compatibility with the precept, or even supplemental status to the practice, depends on how psychedelic use is undertaken.

I think you hit the nail on the head with the heedlessness.

While psychedelics can be used recklessly, usually not for long. They tend to be pretty self-correcting and to lead to deeper levels of self-examination, even when not approached with that specific intention.

A lot of Buddhist practitioners were led to the path via their previous experiences with psychedelics, and I would agree that they can be an excellent adjunct to the practice.

You may be interested in the books Zig Zag Zen and Secret Drugs of Buddhism.

I’ve read Zig Zag Zen, but not Secret Drugs of Buddhism. Thanks for the recommendation

In Vajrayana Buddhism, mind altering substances were traditionally used ( and probably still are in some places ) as aid in meditation.

Yeshe Tsogyal, the prime disciple of Padmasambhava, describes in her book "Sky Dancer" meditating after consuming some Amanita Muscaria...

Yes, but Vajrayana Buddhism is a very idiosyncratic branch of Buddhism, I’m more speaking to the traditions outside of Tibet where the precepts were a little more rigid.