First, if I ever recreated this port, I would rename it to sagemath. Also, if someone else wants to formally take over as maintainer, I am completely fine with that.
Next, when I created the original port I tried to follow their original vision of having the port build its own dependencies. But it was very difficult to figure out how to apply patches to tarballs. Hence the patches like patch-build_pkgs_*_spkg-install. Also, I had to find the patches applied to the freebsd port, and transfer them to the sage port. This was very time consuming. Transferring the patches from python was by far the hardest. Not all the patches applied cleanly, or were even appropriate, and I had to go through the patches one by one.
The reason I stopped actively maintaining this port is that it used to only use python2. Then it started using python3 as well. I just didn't have enough time. I always planned to take it up again, but I never did.
So if you can make it depend upon existing FreeBSD ports, that would really make things a lot easier.