Eh, if you're playing with that revenge gold multiplier, you can always do one of my go-to approaches to that any sort of spelunking conundrums that happen before you have easy/easier ways to deal: Throw sticky dynamite at walls until you find something interesting. Can do it without the multiplier, too, it's just harder to sustain/start up.
Entirely applicable (if maybe messy, but I honestly find the aesthetics of bomb mining to look better than little 2 or 3 tile tunnels burrowing through everything... also much easier to navigate through later, if needed) until you get the giga impact drill from the blacksmith (which has a right click that fires no-ammo block destroying rockets -- neat trick, by the way, if you do a quick left click drill before firing the rocket, you'll put out two instead of one) and/or enough mining speed you're a discount end-game mining mount whatever it is.
If you're handling invasions already, you're probably good to mine up some meteorite, too, in which case I can but repeat the mention of the rocket punch. Pre-wall it's easily one of the best illumination items in the game with these mods running, and can make figuring out where/which direction to dig a lot easier. Its sparks go furthest downward, unsurprisingly enough, but it's got pretty decent horizontal coverage, for what it is.