I don’t bother with planes past WW2, took me ages to level a single nation to Rank VIII and it’s too deadly to be fun, much worse than Rank VIII ground battle which also is no favorite.
For pure grinding I like 10.7 Germany, the leopard 2A4 is well balanced and super effective in the right hands, for casual play usually 6.7-7.0 Germany because Tiger 2 and for fun and chaos 2.0 or under because I can score 22 kill games. I also play US ground a lot, usually around 5.7, and it’s fun but since you can encounter Ho Ris and the Tiger 2 I wouldn’t recomend it unless you are a weakspot god.
I dont get the idea, that every single vehicle should have an equal fighting chance. It puts time travellers like ex.: PT-76 into mid WW2 period and breaks the immersion. Move it to 7.3+ and make it suffer. Some players love to struggle and overcome the challenges.
And there are thousands of vehicles to choose from, it will not affect lineups and queue times.