Hey there, good to see more people willing to go all cavalry
Personally I love cavalry, it really suits my play style being able to use mobility to run rings around enemies before they can reach me, picking my ground before engaging and generally charging in on horse back.
Your lists thus far have looked fine I'd suggest field testing some of them and getting a feel for what you think is missing.
At 500pts most cavalry armies always feel like they're missing something, its just difficult fitting in all the tools you need for the job at that level.
The most essential part though is to have some form of Might pts. I would say that 5-6 is about the minimum you can get away with.
Erkenbrand is really great for this as he brings so much for his pts cost including his horn and 3 might.
Royal guard are still a good call for their higher defence and auto pass on courage. Yes you have Erkenbrand's horn but, what happens if Erkenbrand dies? (This is also why royal guards should bodyguard someone other than Erkenbrand) and ringwraiths will make the horn a mere +1 (had to fight a fellbeast army recently :S).
The royal guard are 2pts more than a rider, you gain +1 defence, bodyguard and lose a bow. Worth it in my opinion.
Sons of Eorl are not units I take for fluff reasons but they are excellent fighters, great defence and two attacks even when charged, on the charge they are lethal. Perfect shock troops.
Your basic rider is a beauty, comes in with all the tools you need, cheap has a shield and bow and can get the benefit of both. If you've taken Erkenbrand the redshield is one point more and worth it.
Throwing spears work for me as they are great for harassing infantry and can make the difference.
Banners Definitely work for me! To field one will cost you two riders but taking them helps you win to the death but more importantly if a line of riders are charged then having a banner behind them gives them an extra roll to win the combat (almost like they had charged) while this does nothing for you in terms of killing models that turn it will help keep riders alive and this is crucial on the turns where you lose priority.
Overall, Rohan have the most versatile cavalry but you need to play it in just the right way. The rules for cavalry vary on opposition, outshoot and soften up the enemy if you'll get messed up in combat but if facing a force that has superior archery you may need to engage quickly.
The only way to learn is to try it. Get onto the field and see what works for you