TT/Tri bikes lot faster on flat roads/rolling hills but handling not as good due to steep angles so if doing very technical road rides not the best.
Having done a couple of IM tri’s would say it depends. If you are mainly going to do shorter events and one IM get a road bike. If you want to do loads of mid/long distance tri’s a time trial setup is more comfortable and quicker over long distances when hammering solo as most races are non draft.
Or as mentioned above get a cervelo with the rotatin seatpost – can then change the seat angle.
Fast descents on a steep angle tri bike are interesting….