+1 for a two light solution with one on the helmet and the other on the bars.
I have a dynamo setup on a 26" drop-bar MTB that doubles for gravel... It has a Shimano hub and Edelux II LED headlight. The dynamo setup is about 6 years old (ancient?) and my newer Light And Motion Urban 1000 headlights are far far brighter. Yes it's great being able to grab a bike and go without having to worry about charging lights but the new LED headlights keep going down in price while battery life and light output continue to improve.
I prefer name brand lights: Light And Motion, Nite Rider, Cygolite. For my night riding I'll usually run two lights on the low setting and only move to medium or high when in more technical terrain or moving fast. I also prefer having a wide beam on the bars and a narrower (spot) on my helmet, but that's just a personal preference for my riding. For example, some friends who do technical mountain biking at night run much brighter lights (1,500 to 2,000 lumen) and need 2+ hours of run time, and they all run a two light setup of helmet & bars.
If you're needing a lot of light for 4+ hours then I'd suggest a looking at systems which meet your run time when set at maximum output but will put out ample light at half power. This will provide ensure you won't be left out in the dark so to speak as cold weather can significantly reduce battery life. Also note that light output of some systems will decrease more than other systems as battery power decreases. Better designed systems will have less reduction in light output for the duration of the battery.
Should you get serious about a dynamo setup, check out Peter White Cycles. He has a lot of good info and sells all the good stuff!
https://www.peterwhitecycles.com/Shimano3N70.php