According to this article you should have gotten it on your first use of the amiibo. And if you did not put Epona into a stable you may not get her to summon again. It does not seem that there are any requirements for getting her to summon other than using the amiibo and it being the first time doing so, there are no indicators that using it at a specific point in the game changes how this works.
If the player has the Smash Bros. Series Link Amiibo, you can use the
Amiibo Rune to tap Link onto the NFC reader. While most other Amiibo
will summon a supply drop of items - the first time the Smash. Bros
Series Link is used, Epona will be summoned instead.
So my guess is, based off their explanation, is that you potentially summoned her and didn't notice and it may be stuck where you can't summon her again. Or you summoned her in the Plateau and you did not get to put her in a stable, thus she disappears.
However, if you wish to keep and ride Epona - you must register the
horse at a stable as soon as possible. If you summon Epona in a place
where you cannot get to a Stable, like on the Great Plateau, you will
be unable to summon Epona again, as using the Smash. Bros Series Link
Amiibo on other days will only summon standard supply drop items.
However; there is another article that mentions a fountain that may be of help to you.
The downside, of course, is that if Epona dies -- that's it. You
cannot respawn Epona by tapping the amiibo again after she dies, and
you will only be able to get her by restarting your game or by finding
the Horse God's Great Fairy Fountain.
So perhaps there is a chance you can get Epona back, based off of what that article says:
Unlocking this Great Fairy will allow you to bring any horse back to
life -- provided that it was registered, and that you have the means
to pay for it. Even legendary or rare horses can be brought back to
life at this fountain -- including Epona (provided you already
summoned her with an amiibo).
But since it doesn't seem like you registered her if she did spawn by chance, it may not work either. The only solution remaining after that, as far as I can tell, is restarting, which I can't imagine you want to do.
Best Answer
Method 1: Stealth
According to this guide, one strategy is to simply try to sneak up on the horses, and try to catch them in this way. If the horse is special in some way (eg the Giant Horse), using a meal or an elixir that temporarily helps improve stealth may help to capture the horse (eg see the Sneaky Elixir here). As you mentioned in your question apples can also be used to help woo horses, though some stealth may still be required.
You mentioned in your question this method can take a lot of time. I found this video showing how to use this method quickly - you'll note that the person actually fails catching a horse on his first try. This is OK, just keep trying and it will eventually work. Also try to approach from behind - this will ensure the horses don't notice you, and maximize your chances of catching one. You can also try using some armour that boosts your stealth.
Method 2: Bokoblins
Another method described here for catching a horse is to steal one from a mounted Bokoblin. While these horses may not have maximum bond with Link, they will be fairly easy to tame. Simply knock a Bokoblin off of its horse, and mount the horse to catch it.
Method 3: Paragliding
Finally, in my experience, you can also try to use your paraglider to drop on them from above. I found this to be a bit tricky to do (since you have to land perfectly on the horse), however if you are having problems with sneaking this method could work, since the horses tend not to notice when you're landing on them from above. Importantly, this method has one drawback: it robs you of your stamina, which you need to tame the horse. Since paragliding uses up stamina, and taming a horse also requires stamina, this method may not be feasible if you haven't already upgraded your stamina.
This video shows the general paragliding strategy, though as you can see, landing it correctly can be difficult.