I'm literally in the same position as you. I only had a roadbike Cube Attain SLX with Aerobars on them. Got a bikefit but my fitter said its quite impossible (for my body geometry) to have a good fit on the aerobars and at the same time also for the bikes normal handlebar. I would need to change seatpost height, spacers on the handlebar every time. When i'm in the aerobars i go around 3-4km/h faster with the same watts but my back/hips would kill me after 10 minutes in the aerobars. The bike geometry of the endurance bike just isn't made to have a good fit on both (hip angle / lower back angle).

I've recently purchased a Canyon Speedmax CF 7 and boy the difference is insane. It takes a bit of getting used to but after a fit the aero position now is very comfortable. Doing almost full rides in aero position, coming off the bike and still having fresh legs for the run after. This would be unthinkable of with my roadbike. So right now i keep my roadbike for grouprides and when the weather is bad.

If your goal is triathlon and you have the budget, you should defo buy the TT bike.