There is no “magic solution”…i am a bit ahead (i have some 7.0+ lineups) and i also noticed everything became much harder…
The other players are better (on average) and everything costs more…so it takes longer.
I simply “accepted” vehicles take longer to research and i actually started taking more time to learn the vehicles, since i now have to use them longer…
I also went back and started unlocking cammos…it is a reason to player lower BRs and it is actually fun.
Also…you can play other nations…and take them to 6.0 as well…
I know this is NOT what you were asking…but freemium game models imply an increasing cost to progress…
You can buy premium to increase rewards…but sooner or later you will run into a similar issue…and it costs money :)
People also say getting better…(more kills less deaths) will make a big difference AND IT REALLY DOES (i can see it on games i do well)…but i personally reached my “skill limit”…so i don’t do so well on most games…