Adding a dissenting voice here. You do not "need" a mage / cleric / any one role in a game based on the 5e system. The difference between a party of 4 and party of 5 is massive and will trivialize most encounters in a way that cannot be fixed by simply adding more HP. A party of 6 in this game would be preposterous.
You can heal by eating food (let alone by resting essentially at any time for free), and the game is much more combat focused than a traditional session of D&D where skills and utility spells are not nearly as significant. There is much less need here for a balanced party that covers all possible bases than there is in D&D 5e, which is already very forgiving in terms of party composition needs.
Literally none of the objections you are posting is actually related to the mechanic of having six party members. With the only possible exception of "needing to rebalance encounters" which as already said at least a dozen times is a bogus complaint since most of them are not finalized anyway and it will take a lot of rebalancing in any case. .
Party control in Baldur's Gate 3 is a complete mess that begs to be addressed. SAY NO TO THE TOILET CHAIN
