I agree - days don't matter the least, what you do with them does however. It's true about the self-labelling as well I'd think; I've seen multiple 5-day raiding guilds that are hardcore for example, but they end up in the world 500+ mark or worse.
But in the end I'd suppose that the labels really are... whatever it means for that guild specifically, it's more of what they perceive as hardcore that makes them call themselves hardcore whilst someone else might have another opinion (like me commenting on the 5-day guilds above).
For us we're a 3-day raiding guild (12h/week) that identify themselves as semi-hardcore due to the progress we push on such low raiding time; we do one split heroic with alts which is baked into normal raiding days and we never ever extend to a 4th day. With this we still made it to top 25 world with our mythic Helya kill and we're sitting on 4 mythic NH as of now with one more day to go.
So, for me, semi-hardcore in my situation is fast and efficient with results, laid-back attitude without spending extra days on a million of split runs.
For the OP;
it's been 3 months, if your heart desires more, go for it. This is exactly what I did; years back I was in a guild only capable of the lower raiding with some one-off heroic (pre-mythic) bosses. I wanted more. So I moved on.
The best advice I can give you is that you should REALLY make sure how the guild seems before making the leap, because if you're blindly jumping from guild to guild many more serious guilds might notice that you're more or less guild hopping a lot - which gives a sense of doubt of "is this guy just gonna jump again soon?".
Throughout the years a handful of guild-hopper people have joined the guilds I'm in to get a chance - none of them have ever stayed for one reason or another. I've been in three guilds for my more high-end raiding and I've been in each guild at least a year on average before moving. This results in me showing that I'm reliable and serious and have planned where I'm going.
Before you even apply to a guild research it properly - check their Warcraftlogs, check if their roster has a lot of movement (constant new people and those people disappearing, look far back with logs), check if they really are as many days as their claim (this one really irks me) and, ofc, their WoWprogress page.
See what their goals are and if they match yours, check out their forum if they have one - check out some previous streams to see/hear how they interact and what the atmosphere is.
Once you feel confident they seem somewhat like a good match add btags from WoWprogress guild page and have a chat with an officer - eventually if all feels good make a real application.
And if and when you make that application make sure to NOT lie about anything. Be open and honest. Nothing is worse than lying from the get-go. I see a lot of clear lying in applications and they just leave a bad taste in your mouth and you lose interest instantly.
What I'm proposing is -ridiculously- much work but in the end I promise it will be worth it. I absolutely hate changing guilds which I do extremely rarely and only if there truly is a pressing matter I can't get away from - mostly because of the work I put into it. Awkwardness while getting used to the new way of a new group of people and put names for all the voices on TS is a whole other deal.
But work gives results. Take changing guilds seriously and take the guild seriously and you'll be good. Even if you get declined just try again even if the task seems just too much to bear with to find yet another somewhat suitable guild. I can't stress enough how much the work is worth it; just remember you're looking for a permanent home and not a motel on the side of the highway to sleep in while you continue down the road to the next one.
Also make sure that your btag is present on your personal WoWprogress page and that you've set yourself to "looking for guild"; I know a lot of people recruit through this method.
As a side-note; as much as it pains me to say it... if you truly want to do mythic, don't join guilds in trade "preparing" for mythic. It never happens. Guilds that advertise in trade are generally... well, let's just say ignore trade chat and focus on WoWprogress.