I would really caution you about substitutes, at least until you have completed "setting" your vagina. Because you have to change out dilators, and the most is about 15-20 minutes, it really doesn't make sense not to use what the surgeon gives you or a harness. Also, there is a method to the madness of the sizes and you really don't need to make it more complicated than it is. There is a point at which you may be using three different sizes within half an hour. It would be a bother to reset a harness. You also do not want to "push" your limit with anything unforgiving. With your hand, you know when to stop, and also when to gently keep pushing -- every session may be slightly different that first year.
Over time, as the frequency lessens, it's not as much bother. First two months are so really are, but for part of that you are healing anyway. Nothing will make it "pleasureable" while you are still stretching to the largest dilator, and the surgeons really do NOT want you to be using toys.
You do not want to use a strap on too early or anything too big, and you also do not want to risk rupturing anything. If your partner can't feel the limit, you will be on edge and may not enjoy the encounter.