Bad luck Brave units

Hey man i know the struggle my Brave Roy is the exact same. So I built him like this. His C skill is going to be fortify calvary. For me Fury was a must or LnD if you have that to help his ATK but also putting him on horse emblem will help as well. Giving him moonbow or luna could also help deal more outright damage and since the quicken pulse seal is a fairly easy reward to get this time around that could make him even better. You could also make him your support character giving him a boost in his stats as well. All in all its not that bad since there are lots of ways to deal with the bane.

Your Lyn isn't totally hopeless actually, she can still reach 36 spd with Mulagir's boost and that spd is just one point away from a +spd tharja
With Swift Sparrow, she can still reaches 40 spd on initiation, and of course that doesn't include cavalry buffs, though that makes her somewhat reliant on cavalry buffs to come close to neutral spd Lyn's potential, but she can still double majority of the cast, really (because mulagir neutralize mages' buffs, so you don't need to worry about them overthrowing your Lyn), +res is just making her even bulkier against mages, so, it isn't harmful

Roy though he isn't totally helpless, he will need cavalry buffs even more than Lyn ever will, that's not his only way of salvation, but that is the least harmful way to save him from that crippling -atk bane

So, easiest way to fix their banes is to just stuff them into horse emblem team. Though you can also try giving them skills like Death or Darting Blow to cover their banes at least on initiation, but IMO, cavalry buffs will fix it better AND open your A slot for more useful skills

Hope this helps

In addition to what others have said, if you really like the character you can S-support them and give them the appropriate sacred seal to neutralize their bad IV. Also, Olivia's a great unit, so if you have piles of her, why not 4*+5 her?

I got a camilla at 4% and I'm at 4.75% on the banner again so you shouldn't really complain too hard about bad IVs. At least you got the characters in the banner at all. IVs can be built against especially since both are horses and get a +2 boost automatically with buffs.
Just get creative, fury automatically fixes any IV if you want to go that route and Life and Death will make an already glassy Lyn glassier but repair her speed. Honesly her normal skill helps with the speed so it isn't too much of an issue.