Training for 1500km Ultra Gravel Race taking palace in June


I’d like to get some feedback and opinions on training for this type of event.
Rider profile:

Age: 37
Xert data:
TP - 280W
HIE - 29-33
LTP - 204w
3h power - 233W

Ride profile:
1500km; mainly flat with some hills; self-supported

Ride goal: to average 400km a day (goal time 4 days). Rather push the average speed and get some sleep, not to be a 15kmh allnighter zombi-rider.

I have done similar events before and ultra cycling is not new to me.
Did 1500km (10k climbing m) bikepacking event in 6 days with full gear and 4-6h a sleep per night.
Did 24h on Zwift (3in1 - vEveresting; Rapha Festive 500; 24h ride time), took 26h total.
And some other smaller events.

I moved fulltime from Trainer Road to Xert as I felt that TR Sweetspot plans burned me through.

So but back to my initial question.
How would you build a plan for an event like this?

What would be the core workouts to hit the ride goals mentioned before?

All feedback is welcome!