Here are some other alternatives (some internally use free Wolfram engine):

Reimplementation in Rust: https://github.com/ad-si/Woxi

WLJS Notebook: https://wljs.io

VS Code extension: https://github.com/vanbaalon/wolfbook

According to https://github.com/rawbytess/hissab, it's not even close to being an alternative to Wolfram. Hissab is described as "A strict, unit-aware natural-language calculator" and its syntax looks nothing like Wolfram. It reminds me of Wolfram Alpha, though.