1. Get exp badges
You can buy exp badges (40% exp boost) in the late game.
I'm not exactly sure how to obtain it, but there's also an Ultimate Badge that provides an 80% experience bonus. I assume it's obtainable in the late game, either as a mission reward or through a world map loot.

2. Other exp bonus
There are upgrades provide better exp.
If you clear the late-game missions while earning all stars, the reward is a 100% EXP boost.

3. Fastest way to grind
Get at least 6 exp badges for 2 crew members.
Go to Angler's reef and find a mission named 'it's a trap'.
The fastest way is to have two characters with all three item slots filled with badges and repeatedly clear that mission.
After clearing mission, use the 'end day' option in the menu, so you don't have to manually take rest.
However, before using the end day option, you must manually rest at the Angler’s Reef bar at least once. This ensures that when you use the end day option later, it will return you to that bar.

I hope this little tip helps and saves your time.