As a mountain biker/off-road triathlete, I’m sorry to hear that we lost you to the road.
As the others have said, it is beneficial to have a TT bike on the trainer so you can get used to the position. It is very different and your power profile will change from the road bike as well, so you’ll have to do a Full Frontal on the tri bike to get accurate numbers. I would make sure you have the TT bike at least 12 weeks out from your event, although longer is better if possible. Just depends on your finances/budget.
Another couple of things to consider with a TT bike is that the position will affect your legs on the run differently from the road bike when doing bricks. You’ll also need to spend some extra time on bodywork when training on a TT bike more. Your hips are flexed far more than on a road bike, so your hip flexors, psoas, quads, chest (pretty much all of the muscles on the front of your body) will get very tight and need more rolling, stretching, massage, ART, etc… In addition, the muscles on the back side are utilized less, which makes them more prone to holding onto that lengthened state and turning off. In this case, you’ll want to do more activation and strengthening exercises. The SUF strength program would be a great addition to your plan because there is a lot of emphasis on the posterior chain- hamstrings, glutes, and core.
Unfortunately, I don’t know any more about switching your profile/metrics, but if you can do it manually, then yes, I’d encourage it. If you’re within 12 weeks of your event, you should be training on the bike you’ll race on. Any farther out isn’t as important, but if you feel especially uncomfortable on it or your power metrics are significantly reduced, then start training on the tri bike as early as possible.
Best of luck to you both!