Honestly, still get the tri bike.
I'm a roadie at heart. But I ride my TT bike a LOT. After you get over the fact you're not riding a road bike for the first month or so, it's like second nature like any other bike.
It's just a skill like riding gravel or MTB or road. Just different. After riding it enough, there is zero convenience my roadie bike offers the TT bike doesn't. Especially since the TT bike has MORE utility. It can have a behind the seat bottle cage for extra liquids instead of using a jersey pocket. It can have a "between the arms" bottle with a freaking straw! It can have an aerodynamic top tube bento box to hold goodies more convenient than a jersey pocket.
What road bike and kit can do that outside of bikepacking?
Tri versus road bike..........you're easily tossing 15min in the trash in a HIM bike leg. Two mph over 2.5 hours? That's five miles plus the inconvenience of nutrition on a road bike versus a tri bike.
Tri bikes are much more ergonomic for the event than road bikes.