I made it halfway through but I kinda wanted to throw in my 2 cents;
I started this year with a gravel bike in march and ended up deciding to do an ironman this past nov20 and finished the bike portion in 6 hours (112 miles). It was mostly flat with slight incline and probably 2500 ft of climbing. I had a hard time keeping my wattage low on the gravel bike when doing centuries leading up to the race and trying to keep up with my road bike/clip on aero and tri bike buddies even slamming my gravel bike stem and adjusting the seat forward to increase seat tube angle (at similar speeds of 18-21mph they were holding 130-150watts while i was pushing 180-220 watts to maintain the same pace). I ended up getting a used tri bike for cheap around 2 weeks before the race and lowered my wattage output by 50-60 watts.
So I did 3 x 112 mile centuries on my gravel bike (22' Trek Checkpoint SL5):
1) 112 miles, 17.7mph avg, 196 watt avg
2) 112 miles 17.8mph avg, 186 watt avg
3) 121 miles, 16.7mph avg, 173 watt avg (our ride ended up being a climb up GMR and 2600ft at mile 70+)
On the tribike Nov 20th 2022
1) 112 miles, 19.0mph avg, 171 watt avg
I tend to like to go fast on flats and I don't mind climbing too much, so I ended up selling the tri bike and getting an aero bike that can act as double duty for being a tri-bike in the future with addition of clip on aerobars although I'll have to figure that out since most aero handbars don't have clip ons for them. I ended up buying the Cannondale Systemsix 2022 aerobike and its pretty damn fast. I'd say go for the aerobike, if I wanted to be comfortable I'd probably just ride my gravel bike.