Been in a LOT of varied guilds in multiple games over the years, from small friend clique guilds to mega-sized world first clans and my views on how to best join a clan/guild/FC have changed over time. There''s a lot of factors in successfully finding a suitable FC but across all that experience two things have been pretty constant. Focused guilds are easier to work with than generalist guilds, guilds who claim to do everything rarely do anything but guilds who are razor focused on like 1 or 2 things "generally" have a higher likelihood of maintaining momentum due to a core group of interested players (be this hard core raiding or house/glam building/RPing, doesn't matter). The second is that large guilds usually have more critical mass and staying power than small guilds, sad but true. I don't mean the mega-church guilds with 400 players all doing their own thing but more the 30-40 people who are all in there for the same reason. While large guilds do have negatives they are usually offset by the fact they take longer to implode and can handle turnover/fatigue.
I know people will have different takes but if you want a sustainable guild I would look for one with solid numbers and facilities and goals, not a tiny guild, or a "social-we-want-to-do-everything" guild. If you can find these two things and are capable of putting up with a few people who aren't "your kinda people" but you can work with in a business-like way then that's a better chance of finding a long term home than most. Most big cross-game clans that seem like life-long bros aren't really a few school buddies, they grow into those clans because the members learn to be more scalable and business-like and BECOME friends as a result of that building process.