That's a good question. Whether getting a custom harmonica as a rank beginner is a good idea or a bad idea depends on your goals. If you don't really have a clear sense of your goals, then this might be an opportunity to think about them.
If you just want to enjoy yourself, a custom harmonica may or may not be something that adds to that.
If you have more specific goals, like making progress (learning bending, scales, keeping time, improvising, etc...), setting time for practice and sticking with it over the next few months, or maybe playing out by a certain date, a custom harmonica would help with that.
Most of the same goals that I can imagine a beginner would have would be helped a lot by taking lessons from a good teacher. There are world-class players that offer live lessons over the internet at reasonable rates. If your goal is to make progress fast, nothing will be more effective than taking some lessons.
If you have a budget in mind for a custom harmonica, I suggest you take at least some of it and get some lessons before you invest in a custom harp.
At that point, I think there is strong evidence that a custom instrument will help you learn faster, it can make practicing more enjoyable and indirectly increase the amount of time you spend practicing, and it can help avoid developing bad habits. Specifically, the diatonic harmonica is most expressive when used with fine motor control.
A stock harmonica requires much more muscle to play than a custom instrument.
It's best to develop fine motor control first, and then develop muscle strength and endurance. If you do it in the other order, most people will develop the wrong muscles and spend a considerable amount of time unlearning the bad habits. It shouldn't take years for a diatonic harmonica player to develop nice tone.
But it's very common because it's hard to learn to relax the muscles of our vocal tract that we don't need to play harp.
Yes, a custom harmonica is a good idea for a beginner, but only if it's part of a more comprehensive plan with specific goals.
I hope that helps!