1. Every guild has problems, some points where they're stuck, some moments when they have roster issues, and generally some dosage of favouritism (never seen a guild where a GM or officers didn't take advantage of being one, either for loot, never sitting out, setting up logs for themselves etc. etc.), as long as it doesn't affect progress tremendously or your ability to raid it's something you get used to and just live with.
2. People on the internet like to bash on each other and shake virtual balls. Simple as. If you ask for help, at best you get advice soaked in "I'm so superior" sauce, at worst you get pointless bashing. Sometimes the advice will be spot on, sometimes it will be wrong. People are people. Take the opinions and do with them what you wish, it's your gametime you decide in the end.
3. Many people came with QQ to the forums only to be debunked in the end as indeed the source of the problem. We had threads like: a guy qqing he afks on trash a bit and got kicked for it, well apparently he afked a lot and on bosses he didn't put much effort either, a guy qqing he got kicked for greeting people on voice comm with "hello my niggahs" and killing people with swapblaster on raid breaks, well, many people said he was annoying and they would kick someone like that as well, another person whining they can't find a guild and then it became apparent this person only wants to come when they're in the mood so your typical "cherry pick raids that suit me" kid, headache of any raid leader, and many many others I can't remember details of but it's often the recurring theme.
I always try to take the depicted situation as "you present this data, based on this data I would do this", however indeed often that data is incomplete or distorted to suit the person making the thread to put them in a victim spotlight. Since it's a recurring theme don't be surprised people are sarcastic, especially since lines with #2 that they like to shake their virtual balls.
However most people told you in the thread here they would leave the guild, so you mostly got advice and not just ridicule like some other threads get.
4. Don't know what's so frightening in the idea people join mythic raiding guild because they mostly care to have a raid team to raid mythic. That's personally what I care to do as well and since I maxed my mainspec artifact now I don't even care to run more m+ except 1 for the weekly chest as I'm tremendously burnt out of it. I don't care to sign a contract with a guild that all my wow time needs to be spent with them and I avoid guilds that demand it. If I want to log 50 alts and farm for big love rocket instead of sitting with guildies, it should be not viewed as frightening or bad.
5. Some guilds are indeed bad and managed in a very poor fashion. People might think your story is fishy, but shit happens. I had a friend that joined a guild that was recruiting on trade for their raid team and they had decent wowprogress status for his aims. It became apparent the guild has no organization, no app process and generally he was told "maybe we'll find a spot for you if you stick around", so it could be they never would. Obviously when he saw that he immediately looked for a place that would offer him a trial spot and not vague promises.
Me myself had bad experiences in the past, for example a guild decided to lay me off after 9 months, even though I wasn't the most problematic member in there, there were people with worse attendance, less skill and bigger dramamakers yet they decided I was the problem. I'm pretty sure if I described the whole story on MMO-champ immediately some people would jump to conclusion I'm lying and for sure I suck so the guild was right to kick me. I decided to do the only thing I could do, just apply somewhere else and keep trying. But at least contrary to your guild that guild told me they no longer want me on the team.
Atm there are tons and tons of guilds that are 3/10 mythic and progressing on something (usually krosus, but some pick aluriel / tichondrius / botanist instead), if you're a decent player you should be able to find something.