Thanks for the reply! I'm in WA, the moonlighting IP law seems very similar to CA. Both my contract and current HR guidance state that any invention is mine if done outside work hours, not using their hardware AND doesn't compete with the company and harms it's business interests - the vague part, especially with a company that has it's fingers in dozens of different software.

The app itself is similar to a module of a much larger product the company sells, but that product is far from being a money-maker for the company (may just break even). People are certainly not buying the whole product because of that module, it's a nice thing to have. There are several other companies that do sell stand-alone apps similar to mine though, there is a market for it.

The other concern was whether starting now the work based on the MIT open-sourced version of my app creates a clean slate for me, since the previous work was done in part during work hours on their PCs. Open-sourcing the app through my employer was the only way not to lose it completely in case I switch companies, and have a chance to try a startup based on it.

Some advice I've got was that forking the OSS version should be done by a friend (not in the company) and I should work on it in the background and officially join later, after I quit - that also feels sketchy.