I rowed at a high college/club level a couple of decades back, some of the guys from my crews went on to worlds and Olympics so got some insight into how they trained. At the time I didn’t know much about training, just did what I was told by the coach (simpler times!), but with hindsight it was amazing how much endurance volume we were putting in for an event that only lasted 6 minutes, and the higher you got the more volume they were doing. Completely consistent with the TR guys telling us how important the aerobic base is even for the high end intensity stuff. Unless you’re a track sprinter/pursuiter the shortest cycling events are still way longer than a rowing race, aerobic base is key. And for something like Kanza it’s everything.
Bearing the above in mind, I’d say that with 5 years to plan it, then if training time isn’t a limiter you’d want to be doing a lot more base/base/base than build/build/build. Maybe alternate between doing blocks of SS base and blocks of traditional base where you do a bunch of 20+ hour weeks with lots of long Z2-3 rides. Build yourself a huge aerobic engine. Add in one block of build->specialty each year to keep yourself fresh and sharpen the top end. Also obviously critical to race Kanza and similar events enough times to absolutely nail tactics, nutrition, equipment, tapering, etc. If you haven’t already, take a look at the thread on how pros train - Pro/Elite training