I love Driller mode in Tetris Online Poland more than anything. It's something challenging that's not multiplayer and where gravity doesn't increase with time.
The problem is that TOP can only be played online and it was down for some time recently, so I'm looking for an offline alternative in case TOP goes offline for a long time in the future.
There is the Dig Challenge mode in Nullpomino, but it's too hard for me. It feels like Dig Challenge mode isn't made for long relaxing but challenging sessions like what I get in TOP's Driller mode. The garbage rising speed increases there, so I can't just casually dig for 15 minutes.