This video by UHaul showed that off-the-shelf trailer harnesses are sold that have connectors that plug right into a Sienna's existing wiring harness. This would be tons better than doing wire splicing myself:
https://m.youtube.com/watch?v=hvLO-HiYEFE
However, a lot of searching revealed that these harnesses are always to 4-wire harnesses ("flat 4"). I've come to the conclusion that this is because there's no easy way to standardize picking up the power wire for the 7-way connector from the battery, the reverse lights are a huge challenge because they're on the hatch but most trailers you'd hook to a Sienna wouldn't have reverse lights anyhow, and the brake controller is something someone would nearly always have to customize. Consequently, most people posting to forums seem to get the 4-wire harness then get a 4-to-7 wire adapter.
Here is the best 4-to-7 system that I've found so far. It has the 3 extra wires stubbed out to butt connectors. It has a nice mounting bracket included. And it's sold in a kit together with a nice 4 wire harness so you know they'll mate together. The instruction sheets are weak but they're available to preview online.
https://www.trailerjacks.com/04-10-Toyota-Sienna-7-Way-RV-Trailer-Wiring-Plug-Prong-Pin-Brake-Control-Ready?language=en¤cy=USD&gclid=CjwKCAjw_o-HBhAsEiwANqYhp0Z0Ly6E_2Hr6V48Nw3tpYa0hWnhopVr6NGZGdlcDHb2Tck6CgJcnhoC8J8QAvD_BwE
Next here are some links to forum posts of people trying to solve this same problem, some of which contain people giving out quite useful info such as specific part numbers for the 12V relay or a brake controller from Curt:
Near the bottom this has a post with some good part numbers: https://www.siennachat.com/threads/7-pin-wiring-ques.1897/
Here's the forum thread answered by madman "Brian" where he cut and spliced two controllers together or something and otherwise did his own wiring. The thread degenerates into a discussion of stranded wire current carrying capacity, so I really should try to read every word in this one even if I was looking for a less labor intensive solution: https://www.siennachat.com/threads/7-pin-wiring-harness-for-2006-sienna.1030/
This thread starts off on fire with a guy giving his entire parts list for his 7-wire install (2012 Sienna though). This thread may have all of the info I need: https://www.etrailer.com/question-83028.html
So, which 4-pole adapter harness is the best? There are a lot of brands out there, including Curt, T-One, and Hopkins.
Here is the Curt harness. Their ad copy makes it sound quite inspired: https://www.etrailer.com/Custom-Fit-Vehicle-Wiring/Toyota/Sienna/2008/C55580.html?VehicleID=20081176415
