They've pivoted away from the consumer market heavily, but you can grab some used LittleBits systems to get the initial absolute basics. Gizmos & Gadgets would be ideal but you could potentially get away with their star wars kit (which people seem desperate to get rid of) and one of their programming boards (microbit and Arduino)

You could then pivot from this by reusing the code you've already done and switch over to custom circuitry.

Am not sure how it compares to Mindstorm but I'd suspect they haven't held their value used in the same way as Lego products tend to.