Definitely tune related issue since you mentioned is custom and not OTS
Go back to the tuner.. Car should feel much faster across the rev range and no errors messages should pop up like that
Check this thread
https://www.supramkv.com/threads/pics-of-supra-with-aftermarket-wheels.2022/page-6
Last post by @Fast_Freddie has a 325 on a 11" rear
Max recommended width is 295 on a 10.5 wheel, 305 would be doable
Beautiful exhaust system, the quality of these are out of this world
Changed the tone dramatically on my ME spec car which sounded very weak from factory
Most Akrapovic exhaust systems are not very loud, they are always balanced in sound and unique due to the nature if titanium mufflers...