Should I Grind Multiple Nations at Once?

Dont you mean up till BR 9?
XM800T and 246 are above 8.0

My top US vehicle is BR8…up to there it works well. M4s and M26s (and relatives) are all good tanks…
BR9 i don’t know…not there yet…

Thanks for all your help, I Am about to reach rank 3 Sweden Air and grinding for rank 2 Germany ground

I played all nations. I played whatever I felt like. My advice would be to not look at it as a grind. Enjoy the vessel/ plane/ tank that you are currently working on. Finding a squadron helps a ton! Playing with friends gives you a reason to hop into a vehicle you haven’t played in a while.
Best of luck to you. Have fun!

If you want variety, go for it!