I have 3k construction bots and most of the time ~2.7k are idle. Just a few actually mine. Power of all roboports should be at max since accumulator charge never deplets fully. Occasonally there are periods of high activity where more than 2k bots mine.
My best guess is that ist somethinig to do with low richness ore, im playing dangeOreus and many ores are <30 ore. So mabey there is a limit of how many ore tiles get processed?
Also the bots mine in a strange pattern: usually they mine a fresh ore from ~200 down to <30 then on a second pass they mine it completly or mine to 1 and then finish on the third pass.

Edit: Ok, i didnt notice that you just updated it a few days ago. Now it seems to work better :)