'virtual' z axis in isometric tilemap

helloo, making a 2D isometric demo for myself - got an NPC on 'level 1' and a player on 'level 0'. I'd like to be able to store the players virtual z position somewhere and dont know the best way to approach it.

initially though about putting triggers on stairways to be able to set it when going up and down, but that would be a faff if i wanted to make a map with a lot of levels.

in my iso grid i've got my levels already separated out into different gameobjects for the different tilemaps so i think it's something to do with a tilemap collider... but then i could just push myself into the 'wall' and be on a level above. it's only a project for me to learn more so i could do it a janky way but id like to know if theres already best practice for this sort of thing!

my usecase for this is stuff like not being able to interact with an object or NPC on a level above or below you, so you could check to see if the levels are the same before allowing an interaction. If i'm going in totally the wrong direction for that also do let me know!

cheers