Dear brother, linguistically ( لغت کے اعتبار سے) to say Alayhi Salam would be better.
Radhi Allah Anhu, traditionally was said for Sahabi and the salaf. Mainly because in quran the word Radhi Allah Anhu was used for Sahaba, and early scholars would also use the same for the Salaf. However later day scholars then preferred Radhi Allah Anhu for Sahaba and Rehmatullah Alay for any non- Sahabi. ( maintaining the special maqam of sahaba then a regular ummati for the people).
Also radhi Allah anhu came after Sahaba had lived and performed actions. So Allah said he is pleased with them.
We also say Rehmatullahi alay, after the person has gone from this world, so they have also done their action. And we ask Allah to give them his Rehma.
Al-Mahdi , has yet to come, and yet to perform his action. So Rehmatullah alay and RadiAllah anhu, is not suitable, given the way those are used.
Hence we say “May peace be upon him” , who has yet to come. So Alayhi Salam is better. And like you said, it’s most deserving for the prophets because Allah tala has used salam for them as well.
For the use of Imam, there is no doubt Al-Mahdi will be the Imam of muslims. His title is Al-Mahdi, the rightly guided one. Imam, the rightly guided one , is just stretching it too much. Ulema have written since saying Imam Mahdi has become shiaar, a sign, of the ahlul bidda, so there is strong karaha , dislike, in using this.
And remember these are all human endeavors and our ulema have decided on these keeping in mind the proper adab. Best course is to follow them, who had deeper insight.