There are two active Toyota Sienna forums, both under the AutoGuide umbrella. ToyotaNation has a sub board for the Sienna that get's limited traffic, and SiennaChat where I'm also a moderator. Toyota Sienna Forum - siennachat.com
You can post future questions about your 2004 Sienna under the Gen-II Van section, and particularly for your case, under the Problems-Maintenance-Repair section.
About your van....
Your 2004 uses a U151E or F (FWD or AWD) transmission. It's set up for Toyota Type T-IV Transmission Fluid. This is an older, 'heavier' fluid meeting the JWS3309 specification. Don't let anyone sell you on the lighter WS used in the 2007+ vans. While it's the same transmission, the calibrations are different. Which in my opinion also says avoid MaxLife and other lighter fluids meeting the WS spec.
Toyota, per Tech Tip T-TT-0122-11 recommends the use of Flush Machines only in the event of serious grenading, and then only for lines and coolers. Not for preventive maintenance.
Other doc's I've seen express concerns about the use of the solvent cocktail with these recirculating machines. Cleaning a bit of varnish from close tolerance valve body components is good. Freeing up big chunks in not!
Agree with the gentle pan drain and replace 3-4 quarts procedure as a way of upgrading the additives package and removing some circulating contamination. You don't want to shock an old, unmaintained unit.
Your transmission should have a real filter that you might consider changing after some fluid changes loosen up some gunk and chunks. There was some discussion as to whether the 2004 vans use the Filtran filter with the Dacron microfiber element, but I believe that was settled by someone who replaced it.