It depends on size of group. If you’re looking to raid with 20 people or less, Discipline works fine. The reason Holy is generally better in heroic is because Discipline can’t actually blanket the entire group in larger group sizes. So for healer cooldowns, Holy is more valuable there.
The kyrian legendary does nothing to change that. What it does is line up boon of the ascended with your evangelism/spirit shell better.
That said, what’s ‘better suited’ to raiding also depends on what you like:
Discipline takes a while to get your head around in a raid environment. They pretty much do no healing outside of their ramp windows, so you need to know how the fight works, and you need to start blanketing the raid before big damage goes out, then bursting during the damage. This makes them generally harder to play in raid because mechanics can force you to have to pay attention to them while you need to be ramping. Basically, your raid cooldown requires ~15 seconds of prep. Like painsmith - you generally want to ramp for the traps, but you’ll also have to be paying attention to spike waves and spiked balls at that time. So while you’re throwing out PW:S on everyone, and casting radiances you need to be moving and avoiding things. With something like a holy priest, you can just be healing, avoiding mechanics and when the big damage goes off, you just hit divine hymn if its your turn to use your cooldown, rather than worry about those seconds of setup. Basically this class isn’t going to be for everyone, because there’s a learning curve involved and you need to be more prepared for each fight than other healers from your first pulls. It’s a very fun and rewarding class, so I’m not saying you shouldn’t play it - just if you’re going to play it, you need to know what you’re getting into.
If you enjoy resto druid/holy priest, they’re fairly good healers in normal/heroic right now. Resto druid’s going to get even better next week with the convoke legendary. They’re both pretty much just throughput machines, and don’t have a ton of raid utility. With resto you’ll be more mobile, as holy priest generally has to plant their feet and hard cast heal for a lot of the fight.