New Player - Need help with my Horse Emblem setup

There are a lot of good choices here, but some require a lot of time to actually build. Time and feathers for units you might not have.

I'd run Brave Roy, Reinhardt, Cecilia, and Brave Lyn. That's pretty much the most basic Horse Emblem setup (Brave Lyn and Reinhardt).

Roy doesn't need much, if any, investment. Reinhardt would need a little, like Death Blow 3. Brave Lyn could do with her bow, or you could give her Brave Bow. Cecilia could be a Colorless/Blue check, or you could make her into a Gronnblade user thanks to Hone and Fortify Cavalry.

You could also run Eirika, and if you wanted to, use her with her default weapon, or you could, like Cecilia, give her a Bladetome from Tharja to make her even more powerful.

id go with sigurd, even tho hes -atk hes still rly good, b!roy is also rly good, so either one of them would work, but sigurd is tankier. then reinhardt since u got a +atk one. and cecilia, since she can deal with both colorless and blues, aka other b!lyns and reinhardts. and then brave lyn ofc. and there u have a perfect horse emblem.

Go with brave roy with Fury, Reinhardt (Perfect IVs), Gunnthra (She is quite good +12 damage thanks to the debuffs and brave Lyn.

Based on meta and efficency I would use B!Roy (due to IVs)/ Reinhardt/ Gunnthrá/ Lyn or Priscilla (depending on longevity or finishing off enemies quickly). I would promote Reinhardt to 5* so he can unfold his true potential and give him his standard infamous build (Death Blow, Moonbow, Quickened Pulse seal), because there is little reason not to use him, especially when he's +Atk. Your B!Roy has good IVs, even though the -def could cause some trouble, and use him if you like the offensive playstyle because he's good at finishing off enemies and the boon in attack synergizes decently with his legendary weapon. But if you want a more tanky sword cavalier, especially against mages or units that attack from a distance in general, use Sigurd. The boon in resistance is actually not bad, but the bane in attack isn't quite ideal.
In the end, feel free to use any hero that you want. Some heroes may need more investment that others, but all in all, there's no unit that is 'shit'. Some units just have more niche usage and awkward stat lines, but if you like that specific unit that falls into this category then there should be nothing standing in your way to heavily invest into that unit.

Sigurd is a monster. You will want an all purpose tank that can stand up to even Reinhardt. Leave damage to Reinhardt, Blade tome Gunnthra if you can afford it, and Brave Lyn.

Best IVs for
Eirika(partially good IVs)
Reinhardt
Gunnthra

And then I would add in brave lyn.

Brave lyn and gunnthra work really good together.