How do you reduce uneven wear on one side of foam tires? specifically in 1/10 nitro onroad
And uneven wear on the front or rear axle?

Swap tyres left to right.

As for front to rear, that's a bit tricky. Maybe try different shore ratings? Are there optional side pulleys for your car?

Left to right wear is highly dependent on your track layout. less chamber gain is way to reduce the wear if your cone is too big. Also what tires are you currently using?

Im going to assumer you have rear tire wear vs the front. If so, you might be getting too much rear wheel spin. you can pay around with over and under drive to see if that helps you and doesn't drastically affect your setup in a bad way.

what is used in my region is hotrace and contact tires, and I use a mugen mtx7

I run hotrace tires as well 40's all round or 40 front with a 37 rear on my mtx7r. Just depends on the grip that day.

My tires do wear more on the side that sees the most load. As long as the track is gripped up, I get fairly even tire. If not, I kill rear tires.

How bad is the wear difference? It be might be normal for your layout.

All foam tires will wear faster on the outside side

For qualifying (when re-using tires) just switch them around left and right.

For finals, most people will cut the outside tires larger than the inside tires for long finals, so they balance themselves out.

Best you record what the tire wear is after every run, then you can calculate how much the outside vs inside will wear over long finals.

For example, with tourer it could be 64F/66R on the outside, and on the inside it could be 62/64 as an example.

With tourer you should aim for a 2mm split front and rear on the same side.