If you just want some placement information for homeschooling purposes, you could DIY this one using above grade level general achievement tests.
Many homeschoolers use them as a means of gauging academic progress, (and also because in some states, they are mandatory in order to be in compliance with homeschooling law):
F.L.O. has CAT5 tests for several grade levels. This is the only real information that we had for our then six-year old when we approached the school about an advanced placement for her. Her scores were fresh, but clearly at the ceiling, even testing out-of-level (2nd grade). In hindsight, we probably should have gone to the effort/trouble of repeating the exercise with higher grade level tools until we got away from the ceiling.
I undertstand your reservations about spending the time and energy on an expert assessment when it seems that there is little to be gained, nor is is necessarily likely to be very accurate. We had those same concerns regarding our daughter's response to an unfamiliar evaluator or strange surroundings.
