I haven’t done a RB400 but they look rad… My background is in Nordic ski sprint racing, which involves a 3-5min effort done several times over the course of a morning, not terribly dissimilar to the 400. Scott Johnston coached me through my whole career so the training we did then aligns directly with what you’ll find in the UA books.
These events are very aerobic in that they require a high endurance base, but still require a very strong anaerobic capacity and muscular endurance to sustain the required high output. You’re spot-on to be prioritizing a lot of Z2 work as a foundation for the objective.
Other than the early base period I wouldn’t do too much weighted uphill work because it quickly becomes quite non-specific given the added pack weight and the necessarily slow speed at which you’re moving (as compared to race pace).
Running economy is quite important – you have a (albeit short) start in the race wherein you want to be able to run very fast to gain position for the climb itself. Then when you’re moving on the actual climb you need to be able to turn over a fast cadence (even in a hiking mode), something which is best trained first on gradual terrain then moving up in steepness as you get stronger and closer to the event. Think short (10-12sec) strides during an aerobic run, hill sprints on a short, steep climb, and similar.
As you progress the training, moving to more specific duration intensity will be valuable – think about how you can take the target duration and speed and break it into chunks – those are your intervals. Train that race speed in short bursts and gradually decrease the rest over time, such that you move closer and closer to the full event as you approach it on the calendar. Again, balancing this type of training with a big aerobic volume is critical.
Keeping the gym-based ME in the schedule is a great way to support the strength demand without incurring a global fatigue on the body.
Sounds like a great objective to train for – good luck!