How to Summon a Rideable Phantom?

I don't think you can.

In this article is explained how to do it, but you actually need a mod, named Phantom+.

And in this Minecraft feedback post is suggested to make them rideable, so it seems it's not possible yet.

You can in Java. The command for Java would be summon phantom ~ ~1 ~ {Passengers:[{id:"minecraft:minecart",Invulnerable:1b}]} This principle is to spawn the phantom with a minecart as its passenger, this way it always stick to the phantom and would not fall off unless the phantom died, which you can fix by killing the minecart entities if phantom is not detected within a certain radius.

I don't really know about bedrock summon syntax but according to what I know so far you cannot access nbt tags in Bedrock. If all else failed, try execute as phantoms at their positioned and keep teleporting the nearest minecart to them using tp command. It is certainly not the best way but at least it kinda works.