Hi there.
I’m guild leader of a horde-based guild on Tarren Mill, which is a popular, busy realm (but generally doesn’t suffer from queues).
We are a casual raiding guild … although that is not all we are. We have those who don’t raid, but perhaps run M+, or simply collect achievements, transmogs, battle pets, etc.
As far as raiding goes … I’m always a little wary about suggesting us to anyone who has mythic raiding experience, because mythic raiders tend to have certain expectations relating to performance which, as a casual guild, we generally can’t achieve 
We run ONLY Normal and Heroic raids. We start in Normal, work our way to the end, then start on Heroic. We try to get Curve, but cannot guarantee it and we don’t get too stressed if we don’t.
We raid twice a week, Wednesday 21:00 CET to 23:30 CET and Friday 21:30 CET to midnight. As a dps, as long as we’re not full or having healer or any other issues, generally we can be flexible as to when raiders join a raid or leave one … although of course it’s much better and easier to have people stick with us for the full raid night, and we can’t have too many do it 
For healers/tanks of course it’s a little more complicated, but there are a few of us who generally have both dps and healer specs available and can switch on the fly, so can usually accommodate a change in healer numbers mid-raid too.
Raiding with us has no requirements other than gear level. We don’t expect you to pick a character and stick with it for the season, we don’t do any weird loot rules (everyone in the raid has a fair shake at loot), we don’t have any attendance requirements (you can come every time, every other time or once every couple of months, if you wanted).
Relating to M+, we have a reasonable number of people who run those. We have a Mythic Monday where mentored groups go through no-stress M+ to learn tactics and routes with no pressure to time, and we have various channels in our discord where people arrange M+ up to 20’s and sometimes a little beyond.
Our guild members go from early 20’s to retirement age … but most are somewhere in the middle, often with families and responsibilities.
Have a look at our realm post if the above piques your interest, and see if we might be what you’re looking for: