Jumping in late, but it might help to look at exactly why XP was invented in the first place.
In the original D&D boxed set, you got XP for getting gold, and adventures were set in maze-like dungeons. It didn't matter whether you killed the monster or used cunning. It was also an incentive. You don't show, you don't get XP. It's a early customer-rewards, loyalty-based system. You show up, you get a perk. You play the game (as it was originally designed, dungeon crawls), you get a perk. If you show up and play a character, everyone (hopefully) has a good social outing.
When AD&D rolled out, this concept of robbery = experience went away under the idea it was the act of robbery, not the amount stolen, that increased the skill. And, this put an end to an unexpected style of play: players would backstab others, especially in tournament play, to get the gold and get the level. Anti-social play wasn't the point of a roleplay game.
So AD&D changed it to kill monsters & play your class, get XP. Ultimately, it gave the best advice for DMs as to how to dole out XP: "Let experience be your guide." Clever. If your players want a slow game, socializing and so on, then slow down the XP or alter how much it takes to get a level. Fast? Do the opposite. Does this sound more like milestone (level up whenever the heck you want?) Absolutely!
So fast forward to 5E and my "experience" with Out of the Abyss. In the 2nd half, there wasn't anywhere close to the XP amounts required for killing stuff and completing quests to get the finale levels. When you're on a major quest to save the world and you have to wander the Underdark hoping for random encounters to bump a level, it sucks. Milestone for completing a chapter & getting a step closer to stopping the bad guys made more sense. Others might artificially boost the story XP for getting the items, but all you're doing is fudging numbers to get players to a certain level. Which is Milestone.
When I tried PF society play, XP was rewarded for participation (e.g. 3 sessions = 1 level).
In summary, "let experience be your guide." It's a good saying. At its core, XP is a reward system for showing up and playing a certain way. So for a DM, it's not about the math; it's about finding what works and being consistent. If you were so inclined, you could use a "gold for XP" system, giving your players an incentive to explore every nook and cranny of every dungeon and ruin. If you all prefer playing an epic storyline, then perhaps milestone achievements (1 level for every artifact gem you find to close the portal Tiamat has opened..) works. It rewards you for showing and gives your incentive to achieve the storyline goal.
Or, as I'm doing, a hybrid in Curse of Strahd, wherein players get advancement towards levels for completing quests and defeating/solving major bad guys, minor advancement for exploring and finding unique magical items, and major advancement credit for finding major artifacts tied to the primary storyline. In this system, except for any loot, random encounters and many combats aren't worth anything at all. It's okay to run away, avoid, and circumvent, whereas a straight XP system could encourage the party to slaughter every foe they find.