If you play WOTC you can put off story missions quite a long time since you have the Resistance Ring and can reduce the Avatar Project via covert ops quite reliably. Just don't do the final Avatar Autopsy research until you're ready to finish out the game. Once you do that research even though you don't have to do the mission until you're ready, the game won't give you many other things to do.

Long War and Covert Infiltration are both mods that make the game longer, but it's more about stretching out the early and mid-game growth than about having a longer time to play with all the toys after you've finished all research and upgrades.

LWotC and/or "Plot-gated chosen missions" lets you play for as long as you want.

The Long War 2, or even better, the Long War of the Chosen mod greatly enhances the whole campaign by adding more strategic depth and more complexity to the game, so that you can fight for much longer before you get anywhere close to the final mission. And I think in the endgame the missions also will be harder than in vanilla XCOM 2 (or WotC). Also, it won't really force you to ever do any story related missions, you can do them whenever you want to do them.

LWOTC
When you start a new game, you are taken to a 'Select Difficulty' menu. Click on the Toggle Advanced Options View button. There you will find several options, including ones to double the length of the Avatar Project and the mission timers. Takes a lot of the pressure to act fast out of the game.

If you...
Slot the Sabotage resistance order
Do the Covert Actions that reduce Avatar progress
Game the Facility-building system

You'll be able to extend a normal game indefinitely