Rein Main here too, so first thing i dont think its bad to charge after shatter more actually do this combo, shatter - strike- charge in the time you do ur strike u give a little time window to follow up for mates or u see it wouldnt make sense to charge, you just have to get a feeling for this when to charge through exp. second. if enemy Rein charges u charge him too, this way u both fall to ground which is better if enemy rein pins u.
3rd. if playing against enemy rein u have to think of poker, bluff with ur shield let it drop fo a sec an get it up again, this way enemy rein might waste his ult
4th dont forget to press both mouse buttons for more awarness. awarness is a must as rein
5th playing against Orisa is actually pretty funny because most orisa player dont know that Rein can pin her against her drum, so just wait till her mortify is gone and charge
6th u can also do a animation cancel with rein, swing hammer- strike swing hammer- this way u get additonal dmg in close combat
7th even if rein has to be on the ground with team and shield still try to get on highground if the situations allows it and jump down with shatter/charge most ppl wont expect that
8th dont be this rein who sits 24/7 at choke point, press both mouse button look behind u, is the whole team there charge through an opening
thats it for now i guess