Simple option is to make the base ql of the ore have a max bar ql from spiraling.
Ore ql × 10 = max refined from spiraling e.g. 150 ql ore at max can be 1500 ql iron
Ore ql base = level of mine x 20 e.g min ql on level 5 is 100
Random ore ql can still allow 200 ql vien on level one just need luck.
This givea high ql ore market value and as the game progress it will only go up in value. Generally this will slow down spiralling and making mining/trade more important.
Each level of the mine gives a garentee for ql and then with this offer a higher chancs to find good ql but doesn't devalue other levels.
Jock
Consideration to base ore ql to be higher for lower % chance to produce iron.