First and most important: Service the inverter cooling system.
For your question:
I think the best source for general information is the 2006 Highlander Hybrid New Car Features manual, publication NCF282U. You can often find it on EBay (I got mine for about $30. Lots of good info) or can get online access on techinfo.toyota.com for a subscription. $25 for 2 days last time I looked. The factory service manual gives some information on control specifics.
The subscription gives access to the factory service manuals, wiring diagrams, new car features manual, TSB’s, and training material.
Also. there is a separate HiHy sub-forum a little further down in the forums list on the index page.
Look for information on the brake actuator failure issue so you are aware.

 

Also curious if anyone has attempted anything more than fairly well graded dirt or gravel roads? Would I be wasting my time and money doing a lift with some agressive tires?

You'd be wasting time and money on that lift and tires. It's NOT 4WD. It's iDrive.

Yes, traction control sends power to the rear motor when it senses slippage. But, it's very limited power. AND, if both front and rear are slipping, it shuts all power off! And, you can't turn the traction control off. Many new HH owners back in 2006 complained about this when they experienced the no power issue when negotiating snow and ice. Enough so evidently, that in later years, Toyota provided the ability to turn traction control off.

Besides, although I love my 2006 HH which I've had for some sixteen years, it isn't rugged enough for harder, off road use. The suspension isn't designed for it. The clearances are limited. The under carriage isn't that tough. etc.