Then mythic really isn’t for you. Honestly, I think you’re right that mythic cross realm should open sooner, but the 20 man lock out is a definite must. I could see it maybe going down to 15, but flex? No. The content is too challenging for the overwhelming majority of players who attempt it to be able to make it flex. You’d have bosses that would be a joke with 10 players, forcing guilds to sit people, and then bosses that would be a joke with 30 players, which would be the new wall for smaller guilds.
A persistent known number of players allows for far greater tuning, and even with that they don’t always get it right. So no to flex.
Sounds like you guys aren’t interested in actually putting forth the effort into tackling the content. My recommendation is to just pug the first couple bosses for funsies. Once you get past the first 3, difficulty spikes abruptly. Then it sky rockets at council, then it goes off the chart at SLG (and ironically dips a bit at sire). If you’re unwilling to put forth the effort to even create a team… you won’t have what it takes to get much further than the first 3. Just how it is mate.