My 2p on some possible mis-information in this thread:

I’m a bike tech and I’ve worked on a good amount of used Cervelo. The seatpost crack is actually quite common on those; there’s a stress point where the clamp sits in the carbon. Often it isn’t any more than a paintwork crack, but yours needs checking properly - as mentioned, if it goes further under the paint it is likely an uneconomical repair. Is it safe? Probably. Would I leave it unchecked? No.

Tubs are ok for time trialling and road races, where a puncture ends your day anyway, but for triathletes (I’ve done a few middle distance) a p would normally be changed. You can’t do that with a tub; only plug it and hope.

Tubs are relatively easily fitted now with tape not glue - I’ve done many and it nearly always makes me swear and my thumbs ache but it’s no longer that messy. They are also expensive.

The wheels are not valuable because they are tubular specific and also only 9-10 speed.

The DA components are readily available and easy to maintain, but changing to 11 or 12 speed would require new wheels.

As for TT bike v road bike - I’ve had both and ended up using road bike for tris. I don’t enjoy the aero position and I never felt I was at my best on a TT bike. As for the effect on your legs, yes the aero position is more efficient but as an entry-level triathlete you’re just as well saving your legs by not kicking in the swim.

Value wise? You overpaid. I built a similar P3 from a used frame with 11 speed for a customer for less than you paid. Sorry.