Very fun and addictive core loop, the mechanics feel great and instantly understandable and I love the aesthetic and limited resolution. The rock break particles would feel a lot nicer if they were tied to the world space rather than the camera. Also, some sort of confirmation with buying upgrades would be nice, it was a bummer when I accidentally walked over stuff and bought it without realizing.
To echo what Squishy Turtle said, my biggest issue was that the progression wasn't really tied to the end goal, apart from the portal chance upgrades. Getting better and faster at mining didn't necessarily equate to getting closer to finishing the game, it just kinda ends based on pure luck. I think this loop would work a lot better if the portal was located in a fixed position, surrounded by really hard to mine rocks or some sort of obstacle - that way each run I have to make the decision of whether I'm gonna go for the portal or just try and build up my money a bit more. I also agree that the bombs became a bit useless as I got faster, because even with upgraded bombs I could usually mine through stuff a lot quicker than waiting for a bomb to go off.
But overall, this was very well executed, good job!