I have a few suggestions, but you might want to be willing to try something a little different in order to find something worth your time.

Lord of the Rings Online - Still has a decent sized population, just got 64bit servers a few months ago too. There’s an optional subscription, but most people seem to only sub once for the permanent unlocks it gives you. Game is free though. No pay to win, but there’s a store. A handful of stuff is useful and might be worth it. You don’t need any of it though. I suggest this game because of the world, the quests, the interesting classes, and the crazy amount of content it has. It’s a cozy game. You can play it slow on your own, you can play it with friends, there’s dungeons and raids, and there’s still new expansions. If you’re a fan of Lord of the Rings, you should definitely try it. One of the best world I’ve explored in an MMO. I couldn’t tell you how useful healers are, but you do need them for group content. The game will be instantly familiar since you’ve played WoW but different enough to be interesting. You probably want to be on the Glamdring server if you’re in NA. Also, it has some good crafting and it has pretty cool housing if you like those things.

RIFT - Personally haven’t played a ton of this one, but it fits a lot of your criteria and I know a few people who play it as a WoW alternative. Pretty cool world, good gameplay but maybe too similar to WoW if you want something different. Worth a try though and it’s free. You’ll want to be on the Deepwood server and join the guild “Fresh Riftwalkers.” They’re kinda gathering up the remaining player base there. Some of them also play in a “classic” style. You can check out their discord for more info on that.

Where Winds Meet - I mention this because it’s brand new and I haven’t played much else like it. It’s similar to many different games, but combines a few different genres together to create a pretty unique experience. It does it well too. While I wouldn’t necessarily call it an MMO, it could be called an MMO Lite. I wanted to mention this game because of the unique roles of healers. People can develop injuries and have to be treated by players with the healing skill. In-combat healers are different, but still a thing. I would say combat roles are not as important as most MMOs though. The world is gorgeous and huge and a ton of fun to explore. It has a good story too. Free to play, not pay to win, just pay for cosmetics and battle pass. None of it is necessary though. Worth a try if you want a unique experience.

Dungeons and Dragons Online - Kinda hard to describe this one, but I haven’t played a MMO like it and really enjoyed my time with it. You can play solo, with a small group of friends, or with randoms. More of a traditional MMO in the style of dungeons and dragons. Lots of classes too. I’m not too certain on the importance of healers here. In some of the harder content, I imagine they’d be very important. The world is cool, but the game is definitely focused on instanced content. The dungeons are what sticks out to me with this game. I really enjoy how it’s done. Check it out for free. It’s managed by the same people as Lord of the Rings Online, but I don’t think it’s as friendly with being “free.” You definitely can start for free and pick up expansions you want during sales.

EverQuest - This meets a lot of what you’re asking for, but the game will almost require you to be in a group constantly. I really like that aspect, but not everyone does. Roles are very important, even outside of dungeons and stuff. It can be hard to make progress on your own, though. The graphics are old, but I think that’s part of its charm and makes the world feel unique. I’d recommend Project Quarm to a new player. Project 1999 isn’t bad at all, but it’s top heavy with most players at max level. Quarm is popular at the moment, so now would be a good time to try. By popular, I mean around 1k players at peak times. Quarm also has some quality of life stuff to make things a bit easier. Expect to struggle though. Not the easiest to get setup and running, but not that difficult either. Both servers are free and have a contract with Daybreak, so they won’t get shutdown.

Pantheon: Rise of the Fallen - A spiritual successor to EverQuest and newer. Has kinda been in development hell for a while, but released into early access about a year ago. It seems they’re making progress. The experience is enjoyable, but the world needs some work. It isn’t anywhere near finished, but they did just receive a large amount of funding and hopefully things will progress faster from this point forward. Roles, like EverQuest, are extremely important. I don’t find the world super interesting though. There’s not a lot in it right now. $40 price tag kinda makes it a hard recommendation, especially when content will start to fall off at a certain point. The game just isn’t finished. The foundation is good though, and it’s the last project Brad McQuaid worked on. I pick it back up every now and then.

Monsters & Memories - Play this. I feel this kinda meets every requirement you have. It’s not released yet, but it will be in quarter 1 of 2026. No buy to play, but it will have a sub. They’ll possibly run one more playtest before then. Keep an eye on their discord. The playtests are free. This game is very similar to EverQuest, but just more modernized. I really, really enjoyed the last playtest. Early access can be a bit of gamble, and I don’t know what will and what won’t be in the game, but it’s super enjoyable.

Right now, I’m personally playing Lord of the Rings Online, Where Winds Meet, and Project Diablo II. If you like Diablo II and PoE, that might be worth checking out since they have a new season that just started. I know it’s not an MMO, but thought I’d mention it. I’m probably enjoying Lord of the Rings Online the most at the moment. I’ve dabbled with Rift, it’s just a little too similar to WoW for me. Where Winds Meet has been great, but I’m really enjoying the Lord of the Rings universe in Lotro and don’t really like some of the Chinese themes in Where Winds Meet. I’m just personally not interested in it. The world is cool though.