I rolled a 5 star +Spd -Res Roderick and saw that he's not rated all that highly, but he does seem to have perfect IVs.
Should I keep him, or sacrifice him to pass his lance on to my +Spd/-Res Cordelia once I get enough feathers to upgrade her?
It really depends. If you plan to use them outside of movement type teams then Cordelia is the better option based on raw stats. In movement type teams, Cordelia is a strong option but Roderick is actually very useful too (I just finished building a +Attack -Resistance Roderick). So really it just depends on what spot you are trying to fill. Also keep in mind that if you keep it on Roderick, then you'll save 20k feathers.
Cordelia, Tana, and Oscar all can be very effective with the Firesweep Lance.
What other blue cavalry units are you sitting on? Blue is by far the most crowded category atm for lance users, so I don't think you need to worry about sacrificing him too much. I can tell you as someone who once upon a time sacrificed a "perfect IV" Jaffar to move L&D to Summer Gaius, I don't miss him.
Currently my other blue calvalry are, at 4 stars. Jagen (+HP/-Spd) and (+Spd/-Res), Mathilda (-Attack, +Defense), and Peri (Neutral).
At 3 stars: Jagen (+Att/-Res), and a neutral Sully from the recent quest.
The thing is, while I have a +Spd/-Res Cecilia and a neutral Frederick that I could uplevel eventually, I don't have any red cavalry, so attempting to try making a cavalry team seems like it wouldn't go very well, especially since Roderick is my only 5 star horse at the moment.
Meanwhile I got ridiculously lucky on the last banner and got a +Spd/-Res Minerva and a +Att/-Def Palla at 5 stars along with the 4 star +Spd/-Res Cordelia, a 5 star Merric, and... I forgot what the 5th unit was, but it wasn't anything worth noting.
So I do have a good base for a flier team as is, while my Cavalry options are somewhat lacking, especially in terms of good reds.
Don't laugh, but honestly that Peri is perfectly acceptable to start a Horse Emblem team with. She does require a kit replacement, but she'd probably be the go to lance user in Cavalry Emblem for most people if Camus hadn't been a free unit many moons ago.
The main thing hurting her is her poor defensive stat, but you can get it up to 29 via Fortify Cavalry. Also, you could go with a Sapphire Lance build on her if you wanted to for now as a cheap build until you hopefully get an Oscar (or Camus comes back). Given the large amount of Black Knights in Arena right now (and Sapphire Lance is one of the few ways you can survive Black Luna) and the upcoming entry of Sigurd, Sapphire Lance is a bit more attractive of an option than it used to be. Combine it with Death Blow or Fury and Lancebreaker plus horse buffs and she'll be able to duel reds and blue lance users and some blue mages.
Definitely give him to Cordy IMO.
Unless you’re desperately looking for a Firesweep Lance user on your Horse Emblem and don’t have Oscar or even Peri, then giving his weapon over to one of the two people tied as the best users of the weapon, with the perfects IVs to run it on top of that, is much better.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes