-You can do N7 missions whenever. Cronos Station after Horizon is the Point of No Return, while Thessia then Horizon begins the Final Arc (but not a PONR).
N7 missions count as a 'mission' but that normally doesn't matter.
-There are three 'sections' to Citadel DLC. You can do them all at once, or spread out. It feels most bizarre or fancanon 'post-game' if you do them all at once, while it feels most natural if spread out in particular wars. I will describe them. Citadel DLC can only be done after the Citadel Coup but for reasons I'll describe, I'd most of the time suggest doing it after Rannoch, perhaps right after Rannoch.
1)The Story: Shepard goes on a shore leave between missions (something he technically always does, but this is of a relatively relaxing sort), and gets pulled into a conspiracy. This is 4 missions long, chained, with a temporary 'hub'. If you want the most characters involved, you always have Liara, James and EDI, need Javik's DLC recruitment mission done, Garrus alive, Virmire Survivor brought on the Normandy, and Tali picked up from the Rannoch Arc. Have your romance secured, especially if they're a Normandy Character (VS, Liara, Garrus, Tali).
2)The Silversun Strip: After the main story of the DLC, you get prompts to start an apartment party. Don't do it. In fact, save it for the last thing you ever do before Cronos Station, IMO. Get the party supplies, sure, but don't trigger the party. You now get full access to the Silversun Strip hub. You can go to the casino for some funs. Or go to the arcade for some funs with some fluff or combat mod (not required) prizes. Or go to the arena for combat rewards and if you do the sidemissions it provides, a questline with a pretty cool result. You have an apartment that you can customize a little. You can invite characters over to the apartment or go out on dates.
The visits in this section depend on whether you're 'done' the main part of characters' stories in the main game. For example, the latest case is getting Miranda's stuff done on Horizon. If you don't, then she won't be available in Citadel DLC.
The visits also depend on 'breaks' of missions before getting more emails to trigger them. Main story missions (like Thessia) count as missions. Secondary story missions (like Monastery) count as missions. N7 missions count as missions. But also Arena matches count as missions (so you can just rush back and forth if you want to do the DLC all at once).
For best story fluidity, I now consider it very valid to save at least one or two of the optional story missions (like Monastery, Ex-Cerberus, Krogan Arc ones if you saved it for this one) to fill up some of the time. There is also the Leviathan DLC arc, and of course the main missions Thessia and Horizon. You can head back to the Strip every 1-2 missions to 'relax' on top of the more stressful environments of the rest of the main game Citadel.
3)Once everything, EVERYTHING is done, trigger the party. This means all sidemissions. This means all Citadel DLC visits. This means doing everything you want done in the Strip, including the Arena questline if you want. This means all N7 missions. Etc. Back to the Miranda example, you need to keep her alive from Horizon then get her email then have her 2 visits in the DLC. Technically you don't need those two visits, but again, it works better for story fluidity. You can actually save visits for post-Party, I just don't think the feeling of the DLC works better that way.
The main thing to remember with Citadel DLC is that its like a more relaxing layer on top of the stresses Shepard is going through in the main game (+ other DLCs). Having him spend too much time in this DLC feels to me like he's under some bizarre escape from it all, but spreading it out feels to me more that he's just taking some time once in a while, while keeping the Normandy in top shape for the war, to wind down a little. Like Shepard can kill the Rannoch Reaper then consider 'Hey, it really would be a good idea to take that suggestion to check out the Strip', or fight the Banshees in the Monastery and go 'Man, I hated those things, I wonder if I can kill some in the Arena to blow off some steam, maybe meet Jack up since she said she'd be there'.
Then at the end of it all, the Party happens because everyone knows this may be the only chance that so many people Shepard knows can spend time with him and/or together.
After the Party, head off into Cronos Station and then the ending sections.