Originally Posted by Traveler2

I would recommend keeping the MR-03 Mazda MX-5 for that dedicated class and just get some t-plates and wheels/tries to make it handle right. Tons of fun that class!
Also keep the MR-04 Evo2 5600 for the Pro Stock class, and just work on handling with tires and suspension parts.
Buy a new 98mm MR-04 ReadySet for the Box Stock class. The McLaren 12C or F1 GTR, C8 Corvette, 2010 Audi R8 are the currently available bodies that would be good in Box Stock. If money is no option, I would also snag an MR-04 with 911 GT3 RS body for that class since they will be hard to find soon!
Then invest in good AAAs, t-plates, wheels and tires. It doesn't take much to get these cars to handle extremely well. I can smoke most people with fully modded and upgraded cars with a near box stock setup that is properly built (which brings me lots of joy) so resist the urge to go for the bling and every bolt on option. It took me YEARS to learn that myself...

Welcome to the challenge!