I do basically that, but with Clerc's toner. I make a litre of the bulk solution of thiourea, tartaric acid and salt. That keeps for months, though it can grow blobs of microbes over time; this has no effect on the results. When I need the toner, I add 1 drop of 1% gold chloride per ml of the bulk solution. Borax toner might need more than that.
You might also try using minimal amounts of toner. I use about 10ml for a 5x7. Less is possible, depending on the result you want. A flat-bottomed tray on a level surface will make this easy to manage.