+Def/-Hp Reinhardt

If he's all you got, build him. Best I've managed to pul is +res/-Spd, and he still does his job well.

4 star? I'm not sure if it's worth it, but I will tell you that that's the IVs of my 5 star Reinhardt and he still works fine. With 30 defense, he's able to use Bonfire to increase his damage by a nice amount and actually take hits from archers and swords. It works, and if you don't have any other blue mages then it may be worth it, but at the same time it may be better to hold off if you can.

I used a neutral Atk Reinhardt for months before pulling one that's +Atk. The difference is honestly marginal. Neutral Reinhardt still helps a ton.

Build a Raven and Triangle Adept build; Those IVs are perfect for it.

A +Def Reinhardt can run Blárraven+ and he does it better than Male Robin. A +Def Reinhardt is probably the best cavalry Raven user, probably only having Leo as a rival.

You can give Rein double Distant Defense 3 in order to avoid Cancel Affinity. But you should be just fine with Triangle Adept 3 when outside of tier 20 Arena. I totally recommend Raven Rein, it's fun!