Help to build a cavalry team

The buffs being applied to this team aren't very optimal. Hone and Fortify will help this team a lot more. Fortify on Veronica, Hone on Hrid. Perhaps a few buff I like Attack Smoke on Hrid so maybe use it as the seal? Threaten isn't amazing on horses. Also your team lacks movement. Reposition will do good on Hrid and Reinhardt at the very least. Probably Gunnthra too

Aside from the things RoyAhoy said, I have some ideas for fine tuning

Use a spd/def 2 seal on Gunnthra.
Seems wierd, but with it she'll double some speedy units better and be able to take a hit from non-brave archers better.

I'm not sure what role you see for Hríd, but he could use a better seal as well. Threaten isn't going to proc in time.
Either go with the defence skills (distant or close) or go with Heavy Blade and a special like Aether or Galeforce

Edit: In this team I'd go for Distant defence. You don't need the healing from Aether if you have Veronica, and none of your units are that great at baiting mages. Hríd can do it if they're debuffed.

So, to begin with, your Cavalry team looks really strong.
There’s one concern, That Gunnthra has way too much investment into it. Also her weapon is kinda terrible but you have Chilling/Freezing Seal so that is fine. She could do well with Bladetome and Glimmer. And I kinda recommend you to have Hone/Fortify Cavalry on all of your Cavalry units.

Reinhardt is a great addition to your team, even though Chill Res would of been better than Chill Atk and foddering Rally Up Atk+ to Gunnthra was not the greatest of ideas, he could do well with Death Blow 4. (If that’s too much investment then DB3 should do well). Moonbow and QP is the way Reinhardt is built. Luna is only used on units who can quad. Reposition/Draw Back on Fliers and Cavaliers is really essential for getting away from enemies incase you are in a sticky situation.

Hrìd is fine, just give him Bonfire and Reposition and Hone/Fortify Cavalry. So is Veronica.

How well is Luna on Reinhardt?