A couple of issues.
Entry level hydraulics won t automatically provided more braking effort than a properly functioning mechanical system.
Not all forks are able to safely go up a disc size, although the bulk can, so you need to check.
I'd try decent pads, degrease the discs, and high quality compressionless cables, maybe £25 Or so. That may well do it.
If it doesn't I'd look at the bike and decide if it was worth the expense of upgrading to SLX hydros - I've had the Deore ones, and while they performed fine they were unreliable, and once I'd finally identified the seal at fault within the master cylinder Shimano couldn't get me one, so I've an aversion to them. Keep a wide berth of "cheap" hydros, the real bottom end stuff from Avid, Tektro etc.