Seems like current stable dev-java/openjdk has an unsolvable (without third party binaries) dependency loop? How can I get it without running someone else's binaries on my system?
you can use this overlay. https://gitlab.com/stikonas/gentoo-bootstrap/-/tree/master/dev-java/openjdk it should get you all the way up to 17, we recently did it on ppc64big-endian even. it also provides rust bootstrap for rust with mrustc (non-ppc64 only) no plans to provide full bootstrap path in ::gentoo though due to different reasons, portage resolver, security bugs and maintenance burden.