A compromise I've seen (and what I do with my hobby projects) is to publish source code to github without a FOSS license, so at least the code I write has some value to others to look at, binary assets (including third party) are included in the project over git LFS so its possible to deny access to those by not providing LFS credentials.
An example along the lines is a project I follow a lot, the developer publishes their core gameplay mechanics code as MIT[1] and it's been very useful to me in a UE4 context (for things like understanding good practices re: building larger systems in UE4).
I guess I mean realistically impossible for you to release projects as FOSS but IMO there is still value if you can push any content you do have ownership to in an open way (at least for educational purposes for others).