I need a strong mage

I'd suggest you wait. But if that's difficult, you can always promote Reinhardt so that you unlock Dire Thunder, which can be given to a 4* +atk Reinhardt in the future. It is much cheaper to make 4*Reinhardt at +10 without sacrificing much of his power.

With Raurblade, -Atk doesn't actually matter all that much since she does a lot of damage with buffs, speeds all you want with Blade users.

Reinhardt basically has neutral IVs which is still good, he might miss out on an occasional KO or two but he's still a powerhouse regardless.

Robin(F) isn't really an offensive mage, she's meant to be used with Gronnraven and TA as a blue and colourless check, same as Robin(M).

So all in all, Reinhardt works best with minimal team support, requiring only a single Hone Attack or Cavalry to reach his full nuking potential. His downside is that he just wants +Atk IVs and nothing else, which yours does lack, though he still hits harder than a truck.
Tharja is the single best red Blade user right now but requires a lot of team support since she needs a lot of buffs to function at her potential. Her -Atk is still very functional but not ideal.
Finally, if you need a counter to blues and colourless units, Robin(F) does that job very well. Her downside is that she has average stats all around and can't reliably handle Reinhardt and Brave Lyn which are two powerful threats right now.

Edit: the other's are right as well, unless you're like me and like merging at 5 star, just wait till you have good IVs for Tharja and Rein.