Thanks!
Update at close to 305,000km (21,000km of ownership)...
1st and 2nd gear has suddenly started to intermittently slip when cold. I thought it may have been low ATF so I decided to do a drain and fill and make sure the ATF was at the high mark when hot, but it still slips when below operating temperature. I'm going to take apart the solenoid screens (filers) this weekend, clean if necessary, and hopefully that cures it. The solenoids may not be getting adequate pressure to lock up. I'm also thinking maybe the solenoids themselves have gone, so that's another thing to look at. Otherwise, my Odyssey may officially enter "do not spend another dime" status. With how little I've spent on it and how much it's gotten me around for the last 19 months, it owes me nothing at this point. Drive it to the ground then scrap it.
I have to note that as I've completely bypassed the radiator and have all the ATF going to a standalone transmission cooler, there's no way for the transmission fluid to warm up unless the van is in gear. I don't know if this has been a detriment to the transmission or not. I didn't want to route the ATF through both the radiator AND the transmission cooler as I wasn't sure if there'd be too much pressure drop for the ATF pump.
3rd and 4th gear still work great so I may restrict this van to road trips only.