I just did some searches and it looks like once you hit the bed rock there isn't anyway to go deeper without getting into the very arcane world of editing the wolrd file directly in a hex editor. Probably not something you really want to get into unless your a really good programmer/hacker.

So looks like your options might be 1. accept the situation and look for other options, maybe a different location the might let you dig deeper enough even if the total distance is greater. Or 2, dive into the valheim data mining forums, learn how the world map file is formated and write an editor of your own.

Sorry to be the bearer of less than joyful tidings :/