I'm a bit Heath Robinson on this, but I would break out the angle grinder to get the rial flat then self level screed/resin.
If black is OK then there are some 14mm thick recycled PVC tiles for £3 each at 50x50cm. Did a double garage a few years ago and have 50m to do soon....but will also add some coloured ones this time.
What ever you use, use as thick as you can find as too thin will lift with hot tyres or stationary turned wheels.....bitter experience. I'll post some pics of the two garages.
