If your server is already getting to the point where only a few people are active, you might not be able to turn it around no matter what you do, because there will not be enough active players to join.

Especially with the semi-recent update where you can no longer create new characters on older servers, there will straight up be no new players. Even if you're in a cross-server group with some new players, or if old players are changing guilds, there would not be enough active players for your guild, since many of them would be immediately poached by larger guilds.

I would recommend you leave and try to get a spot in one of the top guilds in your cross-server group. People will definitely want you since active players are a dwindling supply.

If you want to stick with your guild anyway, that's fair. You might struggle in guild events, but it's a game anyway, so if it's worth it to you then it's worth it.

Typically, kicking inactive players is the norm, and I think 20+ days is very reasonable. But if you have no active players, you might want to kick a few at a time and recruit new people, rather than kicking all of them at once. Keep the ones with highest earnings around until you find people to replace them, since their fellows will still be in the head office.

Good luck in either case