This is one of the major drawbacks of this style of RPG vs those with randomly or procedurally generated levels. If a party member even misses a single session of leveling or questing, they can no longer access that XP or quest rewards. I realize that they want to tightly control player power, but it is not realistic to expect several players in real life to never play unless all original party members are present. The penalties are so high that it strongly discourages players from partying in real life, which is the opposite intent of the D&D phb.
So far I have not played a good RPG with randomly or procedurally generated levels.
Ages ago I finished Diablo 1 and 2 once. Looking back I have no idea why I wasted that time. You walk through random areas to click random enemies to death to get random items.
One of the early Elder Scrolls games had a random map. I did not play it.
Personally I prefer having great hand crafted content once than having totally random stuff repeatedly.
I do not know how to make good stories and characters in a random environment.
And I do not like grinding, which means repeatedly kill the same enemies to gain levels.
Fortuanatly BG3 seems to be fully hand crafted.
As for exp, please give every char the same exp, even those not in the party.
Else you feel forced to always use the party members you find first for the rest of the game.