Being proprietary or open source should not affect your marketing at all. If transitioning to open source is primarily for marketing purposes, it is likely to backfire.

Why do you think your target market would change? I would have guessed the target market is the same, so marketing should be straight forward: same demographics but differentiating you from the pack and how it matters.

My approach:

1.Open source + polished README

2.Listed on directories like create.io

3.Added to awesome-rust

4.Did a Show HN post

That's all I've done so far. 60 stars currently.

Are you asking about marketing techniques as in advertising strategies, or are you asking about the underlying business model?

If the former: the README.md is by far the most important artifact, and specifically the first scroll or two. The call to action (installing/trying it out/clicking to your pricing page/whatever) should be clear and upfront and easy.

Having a website that uses popular doc rendering frameworks also help -- people will recognize the look and feel and feel somewhat assured. Tho I suppose a task planning app prolly has docs figured out.

Godspeed, and thanks for sharing your work with the world! We need it, and we appreciate it.