in the debian ecosystem, there was an old program [support discontinued but still works and still in some repos] called Grive [newest was Grive2] . the deb is in the getdeb repo, perhaps the source code is still findable and perhaps it could be built from source to install on Solus...
It is, was a free one, maybe not as many options as insync but 1. its free and 2. got the job done [in fact, I still use it in Ubuntu Budgie]