I used SageMath for my master's thesis in cryptography and group theory. It was awesome! I highly recommended it and its unfortunately very underrated.
It's an amazing bit of kit. But I'm not without criticisms. I feel it could do with the kind of fresh-start overhaul that really isn't practical for an open-source code base of this nature. E.g. move python 2.6 to python3. Also, you have excellent multi-precision support, but many of the algorithms from other modules don't support precision beyond machine lengths.
But really really good. Love it.
Since version 9.0 released in January 2020, SageMath is using Python 3.