This sounds like one of those fondo timed segment deals like GFNS runs in Asheville.
Can you go as slow as you wish in the first part then do the TT? The GFNS series stuff only has a minimum speed of advancement to each segment that's timed.
That's probably more important than the pacing of the TT itself.
Otherwise, if it is gravel, I stay seated always climbing gravel. Anytime I'm out of saddle I'm slipping tire and wasting my power.
On your power on tarmac I'd think 3mi of 1000ft would be sub 20min and the flattish part around 10min. Either way, you're looking at an effort longer than a 20min "kill it and forget it" effort.
The pacing is that if you can negative split that TT, it's the kind of event to definitely do it on. Less % on the lower grades and more % on the higher grades.
The optimum though isn't splitting it really heavy though. Not like 90% then 120%. It's a lot finer line. Speaking for myself. Like, I'm talking only like 10w difference or so.
Not sure how reliable that is on gravel anyway bumping around.
If possible, certainly easy as possible in the first part of ride. Then, prime the system with a few openers before the TT segment to get the right type of body energy stores activated and ready the second the segment begins. You're warmed up already for sure on the ride, just need to make sure the right energy system is rolling.
(disclaimer, I'm still nowhere near you guy's levels, I just listen to a lot of FastTalk podcasts on way to work).