Unfortunately, the above issue also affects the Nexus 5X. Mir seems to die as it can't load the vendored libraries due to missing symbols. (Copying key parts beneath as the above paste expires in a day.)

cannot locate symbol "atrace_end_body" referenced by "/android/vendor/lib/hw/hwcomposer.msm8992.so"...
[1970-01-01 00:11:54.604946] <information> mirplatform: Found graphics driver: mir:android-caf (version 1.1.0) Support priority: 0
cannot locate symbol "atrace_end_body" referenced by "/android/vendor/lib/hw/hwcomposer.msm8992.so"...
[1970-01-01 00:11:54.613499] <information> mirplatform: Found graphics driver: mir:android (version 1.1.0) Support priority: 0
ERROR: /build/mir-1.2.0+0~20191220231858.24~1.gbp4edda9/src/server/graphics/default_configuration.cpp(172): Throw in function mir::DefaultServerConfiguration::the_graphics_platform()::<lambda()>
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injectorstd::runtime_error >
std::exception::what: Exception while creating graphics platform
ERROR: /build/mir-1.2.0+0~20191220231858.24~1.gbp4edda9/src/platform/graphics/platform_probe.cpp(109): Throw in function std::shared_ptrmir::SharedLibrary mir::graphics::module_for_device(const std::vector<std::shared_ptrmir::SharedLibrary >&, const mir::options::ProgramOption&, const std::shared_ptrmir::ConsoleServices&)
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injectorstd::runtime_error >
std::exception::what: Failed to find platform for current system

The closest explanation I've been able to find is that these functions were added in Oreo, compared to Nougat which the system is currently built with. I would guess to try and downgrade the vendor image to something from 7.1.2, but it should be noted this causes my Nexus 5X to bootloop. Perhaps the 6P will have better luck.