The choices under migration seem to me to be a constantly moving target. I understand completely why the choices need to be limited, but leaving a close voter no option for "other" under migration, to be filled in as text, means to continue one must back out of the APPROPRIATE migration section to a more general OTHER choice. Can OTHER be added to migration options??
We have a mechanism which provides the "other" migration option. Rather than (or in addition to) closing the post, you can make a moderator flag with a custom "other" message (at the very bottom of the list. The message could be along the lines of:
migration candidate for mechanics.SE (vehicle maintenance stack)
I post such flags regularly, and the questions do get migrated sometimes**, so these flags do get processed by the mods.
See also this thread on meta. It's dedicated to expanding migration options.
** Migration is multi-stage process, which depends on a lot of things.