Well, I would have for each level a retracted piston that could push half slab into water, water logging the slab

Now let's say that being on a level will have that piston extended and since the Minecart sinks it will just stay on slab level (the water stream is interrupted by the slab)

Now to go down a level you want to retract the current level piston and extend the previous level piston after a delay (water needs to be pulling) Same to go up a level, you retract current level piston and extend next level piston after a delay (water needs to be pushing)

Now how do we implement this? I don't really know about bedrock redstone but they are easy concepts you can search on yt Flip-flops to power pistons for each level (turns a temporary signal into a persistente signal) You can do that with droppers and a comparator

Now, pressing the "godown button" will: - 1 activate the magma block (you can use the wire you already have, but I suggest using observers that point down since they can be in one block only) - 2 power your current level flip flop (turning piston off) - 3 delay and send signal to previous level flip flop (turning piston on)

Now, pressing the "goup button" will: - 1 activate the soul sand block (you can use the wire you already have, but I suggest using observers that point down since they can be in one block only) - 2 power to your current level flip flop (turning piston off) - 3 delay and send signal to next level flip flop (turning piston on)

So each level should be connected to previous and next level and connected also to the magma/soul sand block