I recruit and help manage a guild that has remained top 100 USA and steadily gotten better.
There are many options you could use here or better yet a combination of both.
I'm not exactly sure what your guild's goals are considering what type of content you focus on and how hard you want to push Heroic/Hardmodes. So any of these suggestions could possibly not apply.
1) People expect "Free Loot Tuesday"? Everyone you would like to have on for Heroic is on? Then do Heroic. If it is your direction as a guild to push Heroic/Hardmodes in some fashion then people shouldn't be dissapointed when they show up Tuesday and find out they are doing ToGC instead of ToC.
2) Don't give people DKP for farm content. Give them DKP for your "progression". Yes I understand that the basic definition of DKP only grants points based on the bosses you kill but you could make this modification. It would be as easy as calculating how much DKP each person usually gets from "farm nights" then dividing that by your "progression nights" then give everyone that amount of DKP even if only 20 ppl show up give them the DKP and don't give any to the no shows. Better yet you could dock DKP for the no shows. Again this is dependent on what your guild wants your attendance rate to be etc.
3) If people just don't want to make an effort to show up and wipe on new stuff when they could get "free loot" then this is a problem. Again I'm not sure how much you want to push progression but even if it's only a couple of hours a night you need like minded raiders that have the same ambitions as you do. YOU NEED TO RECRUIT TO REPLACE. Either do this in private between officers or do it publicly. I tell my raiders in Vent when I'm unhappy with someones attendence or performance. I make it known to everyone that I am actively recruiting to replace them if things don't change. This might be overkill for your guild but it works for mine. EVERYONE CAN BE REPLACED NO MATTER WHO THEY ARE OR WHAT THEY DO. This fact has been proven multiple times. We lost a dozen core raiders on Brutallus and rebounded from it. We lost our main recruitment officer and half a dozen core raiders after killing Firefighter and rebounded from it. As long as the officers are all on board you can replace anyone there is always someone better out there. Even if it's an officer you are replacing there is always someone that would be a better officer in your guild ready to step up.
Again as stated, I have no clue what your guild culture is like, what you expect from your raiders and what they expect from you.
From my personal experience in my own guild, I have a "for the better of my guild" mentality. This obviously is NOT how most guilds are run. This was in place before I took over recruiting and co-raid leading. It was what my members responded to so I had to adapt to this mentality in order to take over when a main officer quit.
You should sit down with your officers and decide what you as officers want. Then go from there. It basically rests on the officers shoulders whether the guild succeeds or fails. Your definition of success and failure is also up to you all.
I hope this isnt TLDR and I wish you the very best of luck. Again I can not stress how important recruiting is. You must recruit. You have to recruit or your guild will die. If you all want to do ToGC and don't have enough people on you have to make it happen and recruit.