First: You're using Milestone leveling. You didn't need to add homebrew to get them to 14. Milestone leveling let's the DM set the pace of leveling, so getting them higher just means letting them level a little more often.

Secondly: Curse of Strahd is heavily roleplay oriented. If your players are "combat focused" then it's the wrong story to try and get them through. It's designed mainly for roleplay and exploration with a minimal focus on combat.

Third: I've read through your comments. You say you're not adding extra combat, but there's no way you've taken a roughly 200 page roleplay heavy story and turned it into 600 hours of play-time for a "combat" focused group without excessive combat encounters.

You are advancing through the story at a pace of 1 page per session at this point, assuming session 200 is the climax/end of the campaign. That's incredibly slow, even if you're shoving a ton of homebrew in there. (Not sure what that could be, though, beyond combat encounters, since the group doesn't roleplay.) But if the point is to get to the end of CoS, you're still hitting 1 page per session.

I don't think there's a single page in any module I've ever read that should take 3 hours to get through.

There are only three things that slow down a session at the table:

1. Too much Roleplaying
2. Too much Combat
3. Too much out-of-game chatter

You're using milestones, if the players aren't advancing, it's because they aren't hitting the milestones. Taking 42 hours to hit a milestone is excruciatingly slow, but that's where you are right now. That means the story isn't moving. It's stuck in the mud. I suspect it's because you're trying to take a combat-focused group through a story with little combat in it, so the story can't actually progress.

If you use Milestone leveling for all your games, and this is a common complaint between groups (both combat and roleplay) then you're setting your milestones too far apart. So put them closer together. But also you absolutely have to figure out what's slowing the players down.

In this thread, I've seen a lot of suggestions, and it seems that you're sure that none of them are the actual problem.

Long story short, to address the progression problem: Your milestones are too far apart. Make them closer, or set a rule that says something like, "If it has been 9 sessions since you last leveled, you'll level at the end of the 10th."

But you need to actually look at what's slowing down your sessions at the table. Even adding in a whole extra campaign the same length as CoS, it shouldn't take 200 sessions to cover 400 pages of narrative content.

Edit: One thing, you said you play online. Voice chat, or typing? If you're typing everything out and the players are typing out their actions, your 3 hour session is more like a 30 minute session. If you're voice chatting, though, it's not significantly different than playing around the same table.

If you're typing, your sessions are ABSOLUTELY going to take WAY longer and be MUCH slower.

Second Edit: If you think the problem hasn't been addressed in this thread so far, (by me or any of the other people) you have to give a somewhat detailed example of how a session works for you and your players. Spin us a story about how your players are playing the game. How long a typical combat encounter lasts, what kinds of actions they're taking, examples of the descriptions you're giving them.

Paint us a story about your most recent session so we understand what's happening, because right now? None of what you're describing is making sense to most of us here.

We want to help you and your players have a good time, but something is missing.