5° is standard (ADA) which is 1:12. So at 8" height you would have an 8' ramp. If this is too long to line up the mower, shorten/steepen it as little as is necessary for you to use it. How steep is too steep is impossible to know without understanding all the circumstances.
Find a solid sloped surface somewhere nearby and test drive the mower up and down, to get a feel for how steep is driveable and safe on your mower.
Some issues to consider: 1) steepness of ramp, 2) structural support, 3) design of bottom edge, 4) mix design.
1) In the U.S., we can’t exceed a ratio of 1:12. That is, 1cm of rise requires 12cm of run. Also, it requires a handrail.
2) Concrete weighs about 150 lbs. per cubic foot. Whatever supports the “concrete floor” will also need to support the concrete ramp.
3) Concrete that comes to a point, will “chip off” over time, especially if wheelchairs roll over it. I’d chip down into the concrete floor (if it’s structurally capable) so you have a 4cm or so edge. (We often place a piece of rebar within 3/4” of the edge too.)
4) we like a “rich” mix for tapered concrete projects. That is to say, a 5 sack or more mix is important. Also, do not use too much water. If it’s too runny it will not keep it’s shape on the slope.
There are many factors in the design and finishing. Here’s a site that may help.
https://www.concreteconstruction.net/how-to/materials/top-10-myths-in-concrete-construction_o
Best Answer
I'm not sure "thickness" really matters as much as the overall design of the ramp. U-Haul trucks come with 12' long aluminum ramps that one person can lift and they are made of pretty thin aluminum. They are rated at over 500lbs because of the overall design of the ramp.
Thickness really only matters when talking about one single unsupported span, and that is dependent on overall design. You could have a 24" unsupported span with 1/4" steel or a 6" span with 1/8" aluminum (all made up numbers).
A better approach would be to find an affordable metal that is suitable for the purpose of a walkway, then decide what design would allow you to use that based on its weight and flexibility. Once that is done, you can look at the cost of a more typical surface (wood decking), compare costs, and see if metal really makes sense.