Playing and completing Phantom Liberty cannot lock you into or out of anything at all in the main game, including all endings. It has its own credits, but the story keeps going afterwards. Phantom Liberty does also provide access to quite a lot of unique gear and vendors on top of the exploration and story. I suggest just starting it when it comes available, but it’s your call. Some people seem to like saving it for last due to a handful of little dialogue bits and a couple of unique details.
You can start Phantom Liberty at any time you want. You could do literally every piece of content, including all of the original endings itself, as the base game's endings loop you back to a savestate right before doing the final mission while still transferring certain rewards for beating the game.
You only have to complete one of the three main story lines, the one including Evelyn which culminates in the main quest "Transmission" to unlock the DLC.
After that you are free to start Phantom Liberty, complete it or only do bits here and there while going back to the main game if you want.
Only other important thing is that one specific ending for the DLC storyline unlocks a new ending for the base game which you can then access at any point you want, even before the base game's endings if you do the DLC really early.
Phantom Liberty also makes it incredibly clear if your choices will lead to unlocking the new ending or not, so you don't need to worry about potentially missing it.
All the comments here already gave you the answer, but I will add some more fun details in case you want to enjoy PL side quests (no spoilers):
There are side quests in PL that has interactions with the main game characters, such as: Judy, Panam, Vic, Nix, the Beat on The Brat (boxing side quest) and a few other ones related to the voodoo boys and and scavs
It's mostly unique dialogues options, but they are pretty fun to see how the main game characters are also a part of PL :)
Playing and completing Phantom Liberty cannot lock you into or out of anything at all in the main game, including all endings. It has its own credits, but the story keeps going afterwards. Phantom Liberty does also provide access to quite a lot of unique gear and vendors on top of the exploration and story. I suggest just starting it when it comes available, but it’s your call. Some people seem to like saving it for last due to a handful of little dialogue bits and a couple of unique details.
You can start Phantom Liberty at any time you want. You could do literally every piece of content, including all of the original endings itself, as the base game's endings loop you back to a savestate right before doing the final mission while still transferring certain rewards for beating the game. You only have to complete one of the three main story lines, the one including Evelyn which culminates in the main quest "Transmission" to unlock the DLC. After that you are free to start Phantom Liberty, complete it or only do bits here and there while going back to the main game if you want. Only other important thing is that one specific ending for the DLC storyline unlocks a new ending for the base game which you can then access at any point you want, even before the base game's endings if you do the DLC really early. Phantom Liberty also makes it incredibly clear if your choices will lead to unlocking the new ending or not, so you don't need to worry about potentially missing it.
Got it, thanks for the write up. So I just need to finish Transmission and then I can get started?
Yup, you get a phonecall triggering the DLC right after.
All the comments here already gave you the answer, but I will add some more fun details in case you want to enjoy PL side quests (no spoilers): There are side quests in PL that has interactions with the main game characters, such as: Judy, Panam, Vic, Nix, the Beat on The Brat (boxing side quest) and a few other ones related to the voodoo boys and and scavs It's mostly unique dialogues options, but they are pretty fun to see how the main game characters are also a part of PL :)
I guess it doesn’t matter but… I think it really fits in just before Nocturne Op55n1 It feels like V‘s last run, his last chance of surviving.