Now I'll have to use Pages app again, I'm not going to pay a subscription service for invoices, it's ridiculous for someone who need makes 1 invoice per month.

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.

Since Slimvoice removed the Google login for privacy reasons, I'm not surprised they've escalated their idea of living off the grid. Their ideals have left us out of their great service, and honestly it doesn't bother me but they should have at least left it self-host.