I'd love to hear if anyone has other alternatives. I'm finding myself in a similar place as you do. I found on the About and Terms pages a few software projects listed, which the author claims are what powers Slimvoice:

  - https://www.blender.org
  - https://www.gimp.org
  - https://golang.org

General tech ^ and libraries:

  - https://github.com/aymerick/douceur
  - https://github.com/jung-kurt/gofpdf
  - https://github.com/lib/pq
  - https://github.com/qiangxue/fasthttp-routing
  - https://github.com/valyala/fasthttp
  - https://github.com/valyala/quicktemplate
  - https://github.com/bamiaux/rez
  - https://github.com/MithrilJS/mithril.js

And lastly, fonts:

  - https://github.com/rsms/inter
  - https://github.com/IBM/plex
  - https://github.com/weiweihuanghuang/Work-Sans

Aggregated from from:

  - https://slimvoice.co/about
  - https://slimvoice.co/terms

Copied URLs just in case those pages go kaput.