Bubble Elevator Block Swapper

Total redstone noob here… trying to build a bubble elevator in one of my underground builds that will take me to the surface, and by the time I get there the block is swapped back to magma for when I’m ready to come down. I’m also trying to hide the piston and magma/soul sand block underground if possible so it’s not on the surface level where the grass is. Not sure if this is possible but thought I’d ask! The tube I’m going up is 20 blocks which is slightly too high for a single button to reset it in time so I need help figuring out a simple clock too. Thanks!