Can Sigma 105mm F2.8 EX macro macro pan/tilt be used for fixed focus? Take portraits at the same time

105mm is a bit reluctant to be a telephoto lens. You can try adding a 1.4X or 2x adapter.

105mm lens is used for portrait photography on C-frame SLR, with a slightly longer focal length, which is ok outdoors, but inconvenient indoors because it is too far away from the subject.

Compared with Sigma's 105mm lens, EF's Canon lens certainly has advantages in main indicators. As long as it is two squares larger, it can provide you with better background blur portrait effect.

If you need portrait photography more, it is right to choose EF 100mm between them, and there is more room for choice between them. ...

And from your consideration of EF lens, you can see that your machine should be Canon, so if there is no special reason, the original machine with the original lens is of course the first choice.