Penpot might be a good alternative for Figma. It is OSS and has plugin support. Prob quite feasible to consider a plugin that utilizes Nostr as backend for comments.

GitHub

GitHub - penpot/penpot-plugins: Repository for developing and running Penpot plugins with examples and libraries.

Repository for developing and running Penpot plugins with examples and libraries. - penpot/penpot-plugins