[ignoramous]

Thanks a lot for being so thorough with the answers. (IANAL either but) Given the above, I think you'd be fine with AGPLv3.

MPLv2 and other related licenses such as Eclipse Public License v2, Erlang Public License, do have the advantage of being well understood and in some cases auto-approved for use at various enterprises and thus a good midway between MIT / Apache and GPLv3.

That said, you'd be right to lean more-copyleft (going Server Side Public License, for example) if K6 is a key product (and not a complementary product), though Bryan Cantrill thinks you might be better off closing up the source in that case: http://dtrace.org/blogs/bmc/2018/12/14/open-source-confronts...