What about just penalizing people hard who use lower stars and ranks? Make a system with a fixed penalization. They sort of do it when you run less than 5 heroes, you get penalized HARD for that, but not when you run a full 5 man team and min/max.
For example a 1k power penalty if there is a star difference, and 1k for every bar difference. Make it scale. So for example Jeff uses a 10* plat Panzer with a bunch of 5 and 6 star silver heroes. He would get penalized 5k points just from the star difference between Panzer and Frencoise, then another 2k for the rank difference. That’s 7k right there. Then add another 6k for the difference with Operator, 4k for Heimlock, and 3k for Ifrit. Jeff would now be facing a team 20k points higher than himself.
Majik isn’t min/maxing so he would only have 3k power added for the difference between his 8 and 7* heroes, and then 2k for the rank difference of Gale and Panzer. His penalty would only be 5k. Average out the difference and Jeff would only face Majik if Jeff was 40k and Majik was 55k. Make skills not add to power and make heroes power more standardized (Night not way below others, for example) and I think things would be in a much better place.
Of course things would need to be fine tuned. Maybe rank difference is a 2k penalty instead of 1k, or each bar is 500 points. I think this would help though because you are penalized more the more you have a disparity in hero power. Meanwhile legitimate players who just havn’t been able to level certain heroes up as fast as others are only marginally penalized.