Watch Brotherhood, not the original.
Wrong - watch the original up until they get to Rush Valley (around ep26 or so IIRC) and then switch over to Brotherhood. Or just watch all of the original and then Brotherhood.
Brotherhood tries to rush the first half of the original into 3 episodes which ruins the pacing.
I mean, if you can enjoy the pacing of Frieren, you can enjoy the pacing of the original FMA.
I obviously don’t want to spoil anything for anyone who hasn’t seen it but the rush in Brotherhood robs some crucial plot points of any emotional impact and is lesser for it.