Stonehenge during Fall Equinox

If I remember correctly, the Druids did a great job of claiming religious significance at Stonehenge (even though there really isn’t any historical data to support that was the use) and are allowed to enter beyond the ropes that keep people back from the stones on those days. I would anticipate extra crowds and some access issues are possible. If you are worried, maybe go for a different date.

The “Druids” have been around since the 1920s. That is the level of connection they have with the builders of Stonehenge.

Although the original Druids were of course around at the same kind of time as Stonehenge was constructed, before being basically wiped out by the Romans.
Yes it is unproveable whether or not the original Druids used Stonehenge for their rituals. It is not unreasonable to conjecture that they "might" have.

"Stonehenge" by Spinal Tap sums things up nicely:

In ancient times,
Hundreds of years before the dawn of history
Lived a strange race of people, the Druids
No one knows who they were or what they were doing
But their legacy remains
Hewn into the living rock, of Stonehenge
Stonehenge! Where the demons dwell
Where the banshees live and they do live well
Stonehenge! Where a man's a man
And the children dance to the Pipes of Pan
Hey!
Stonehenge! 'Tis a magic place
Where the moon doth rise with a dragon's face
Stonehenge! Where the virgins lie
And the prayers of devils fill the midnight sky
And you my love, won't you take my hand?
We'll go back in time to that mystic land
Where the dew drops cry and the cats meow
I will take you there, I will show you how
Oh!
And oh how they danced
The little children of Stonehenge
Beneath the haunted moon
For fear that daybreak might come too soon
And where are they now?
The little people of Stonehenge
And what would they say to us?
If we were here, tonight

We were there a couple of days after the Autumn Equinox last year. There were still many people camping on the road nearby. We were on a Mad Maxx tour and there were many druids and pagans in Avebury.

Edited to add that the additional people added to the ambiance and didn’t interfere.

In 2014, we visited Stonehenge on the Summer Solstice. There were a few pagans celebrating there, but it was by no means a zoo. The next day, we visited Glastonbury Tor and there were pagans there engaged in a ritual dance, but again, it was a small number of people and just made our visit more interesting. I can't imagine that the Autumn Equinox would be different.