I'd go for open source projects. They usually have bigger code bases and good practices, that they enforce on their contributors with code reviews and such.
It's a good way to get feedback on your code, something miss out on personal projects and get much less of in university and corporate projects.
You contribute code to slint under MIT and you can also use that contributed code under MIT or any other license of your choosing, it stays your code after all. You can not use other peoples code from the slint repo under MIT though, that is correct. The royalty free license tries to get as close to MIT as we can while limiting the use on embedded... but with that limitation in place it is of course not an open source license.
Contributing back to Slint is in no way required, so if you do not like our contribution terms, then you are free to not do so. Ypu are also free to use something else if you do not like our license terms.
We try to make all of the terms as clear as possible. We rewrote the Slint licensing page several times, often with extensive community feedback, to get it as clear as it is right now. If you have ideas on how we can improve, I am all ears.