The only option to focus is to change the distance to the subject.

Basically, yes.

Is there any existing workaround?

A. Some rails are indeed one workaround rather than moving the tripod. There are some simpler models, like just a sliding plate on the head, like this one of the same brand.

B. Some other has some sliding bellows for both elements, the lens, and the camera.

C. But you can also use just a sheet of paper if you are on a table for example. Put the camera on a paper, a napkin or some smooth fabric and move it.

D. Another workaround is to move the subject, but of course, it depends on what your subject is.

E. If you are in a garden, one trick is to balance the tripod. Put two legs pointing to the front and the last leg pointing backward. If you extend or contract a bit this leg you will be also moving a bit forward and backward the camera. You need to compensate also the tilt of the head, but you have some more control than moving the tripod.

F. Another way to keep better focusing is by closing the aperture. Yes in some cases you need a great amount of light to compensate. So you probably need to fire a flash, or having some bright beam pointing at your subject. You could use some bright led lamp. Nowadays is easier and safer to have bright lights.

is focus stacking possible with extension tubes?

I suppose you always have some limitations because of the change of perspective, but focus stacking as far as I understand drop the zone that is not in focus. What you could have is some overall deformation.