The correct spoke tension is dictated not by the spokes, but by the rim. Manufacturer recommendations vary from about 90 kgf to 140 kgf. 100 kgf is considered a safe spoke tension for a rim without a specified tension limit.
The bad news, in your case, is that the spoke gauge you have is incorrect for bicycle rims, and you'll suffer chronic spoke loosening unless you glue the nipples in place (in which case you'll suffer other problems). Chinese hub motor manufacturers have opted to appease the moron market by furnishing spokes that don't work with the rims they use.
Bigger isn't better for spokes. The strength of the wheel comes from the rim. A spoke's job is to have the correct amount of elasticity for the rim it's used with, so it can continue to provide support as the rim flexes under load. The heavier you load a given rim, the thinner the spokes must be to accomplish this. Thicker spokes must be turned up much tighter, which means they only work with thick heavy rims like those on motorcycles. Bicycle rims buckle and crack under the tensions that allow thick spokes to work correctly.
In the long run, you'll be much better off if you cut your losses and relace the wheels with 14-15ga butted spokes and sturdy high-quality rims. Short of that, you can apply boiled linseed oil or some other mild thread locker to your spoke threads in an attempt to keep your spokes from unscrewing. It won't keep them from going slack when the wheel is loaded; it won't allow them to provide support to the rim as intended. But it may keep the wheel from self-disassembling.
If you relace with thinner spokes, your will probably need to use small washers (2-2.5mm) under the spoke heads to keep them from burrowing into the hub's oversized holes.