How to reduce shake at high focal lengths {2600mm-3900mm with adapters} when on a tripod?

I had a problem like this while shooting in India with a small folding tripod on a Nikon F3 with a borrowed long lens. What I did (remember this was field fix but it worked great) was use the camera's delayed timer. I focused and framed the shot, and started the timer, by the time the shutter released the camera was steady. Like I said, not pretty but it got the shot. The pictures were razor sharp. Don't hesitate to do whatever it takes. You're a photographer, you're creative. Make it happen. Best of luck.

You can have a shake on the tripod. Yes, a sandbag or some other weight can help you to stabilize the tripod.

You can also reduce the extension on the tripod's legs.

---

You can also have a shake on the camera, even by you pressing the button or the mirror going up. So, try using a remote shutter or a delayed one, probably using some live view function.

---

But you probably have some shake on the lens due to the wind, as you mention.

The only way to go there is to protect the lens from the wind. Try to stand next to the camera, not behind it, hide behind a wall or something like that.

---

When shooting try to underexpose your photo changing the ISO speed, so you have a faster shutter speed. It is easier to remove the resulting noise than a blur.