Target the seasonal challenges, they give heaps of XP.

From any of the above mentioned activities: try also to be on a planet (or helm) with a clan member when you redeem your bounties and shared wisdom buff is active.

Weekly challenges give stupid amounts of XP, and you can optimize them. If something wants you to use scouts a bunch and progresses slowly, and another wants you to play some strikes and crucible, use a scout in there. Grab bounties that complement what you're doing.

I would keep an eye though on some weekly challenges and see how fast they're progressing. It isn't worth doing a challenge that'll take 20 Gambit games if you've finished every other Gambit challenge. Try to get the best bang for your buck. The seasonal story is always a good way to do that

Look through the weekly challenges in the battlepass, and try to do as many simulatenously as you can. It won't take more than maybe a couple of hours of your time since towards the end of a season, you can complete numerous challenges at the same time. I was able to get my wife's account from level 60 to level 97 in about 3-4 hours of grinding on a saturday. Id also pickup bounties from vendors and hold onto them until you just so happen to complete some for a little bonus of xp too.

Whenever you’re going to play, fire up the Destiny 2 mobile app and grab bounties for your character while everything is loading. You can look at the bounties and optimize your time a bit (SMG kills from Banshee and Eris, for example), but honestly, I just play how I want and get a bunch of bounties completed anyway.

If you want to be super optimal, save up bounties and wait to redeem them until you’re in a fireteam and not in orbit. Tower and HELM both work fine. This will let you take advantage of fireteam XP bonuses.

One other rule of bounty grinding, always try to redeem any saved bounties before the next daily/weekly reset if you want to be able to do those bounties again during the next cycle. Sometimes there can be quite a few repeats from day to day, and redeeming before reset keeps your options open.