Even number patches are extremely hard on any endgame guild due to the sheer lack of content and only meaningful endgame being tokens farming and savage content.

Odd number patches there typically is more for a FC to do although honestly still lacking.

Basically all you can do is try to find things to keep ppl active or level alts and help ppl with content using them.

This game endgame progression wise can be completed in about 6hrs of game time a week once you learn the content. Most serious endgame groups/guild I know spend most their times working on Feeding runs to get everyone top parse of FFLogs.

As for keeping members. This game it is harder then most mmos because people do not need guilds at all to progress, so the loyalty factor is missing. You will lose people during the low periods in patches it is inevitable. People will get bored and log in for 1-2/7 days a week do their weekly progression content, other not as caught up or who want to play more then endgame will log mid/end week and say man this guild is dead the leave. Core issue is in game design...nothing you can do about it really. Been fighting it since ARR launched.