Miscellaneous thoughts from me... hopefully you can find a diamond or two in the dunghill.
I had gotten interested in, and briefly tried, polyphasic sleep a few years before doing PBP in 2015. While I didn't stick with it for more than a couple weeks at a time, it gave me some tools that came in handy. It's amazing how much a 5-20 min nap helped when I felt myself getting too drowsy. And getting a 1.5 hour nap in Fougeres on the way out really helped. For this to work, you can't have a lot of caffeine or other stimulants in your system -- I liken it to skipping a stone across a pond.
If nothing else, being able to break up your sleep into 3-4 hour chunks would go a long way toward making this feasible. I'm curious (but won't pry) what medical condition requires sleep in 8+ hour blocks... having a randonneuring friend who can do the ride with you may be something to consider, for safety.
+1 on GhostRider's comment about being efficient when stopped. I wasted countless hours in 2015 on things that shouldn't have been necessary: trying to find the right cable to connect my battery pack to my headlight (I'd packed a USB micro when I needed a USB mini, or vice versa, d'oh!), buying additional batteries for the taillight, finding and buying arm and leg warmers because it was much colder at night than I expected, buying more ibuprofen when I ran out, and trying to find electrolyte supplements when I feared I hadn't gotten enough. And all of these were hampered by my non-existent French-speaking skills.
In 2023, I hope to have more time for sleep thanks to dyno lights, packing lightweight arm and knee warmers (and potentially doing more of my sleeping during the coldest parts of the night), packing more ibuprofen but ideally needing it less, and consuming a more complete stream of nutrition as I go. And speaking better French.
I'm on the fence about trying the 84-hour group versus just going with the 90 again. On one hand, I think I could definitely get the whole thing done a few hours faster (and go back to bed sooner!) with better preparation, but on the other hand a lot of things did go well in 2015 (no flats or crashes during the ride!) and the universe doesn't owe me the same luck next time...