1. My kid started taking showers with me as soon as he could stand. He liked them about equally to baths, and it saves me a lot of time.
2. You're overestimating the difficulty of a deep tub, or how hard it would be to put a plastic tub or inflatable pool or something else in the shower.
3. Most importantly, you're overestimating how long of a period of your life your kid will be unable to more-or-less bath himself or herself. I'm not saying you won't need to supervise and assist, but you don't need to be bent over the tub the whole time.
Basically, you're talking about a major renovation to add a feature that might be useful for only a couple months. This strikes me as about as necessary as getting a built-in Bumbo in your nursery.