I’m considering doing two levels of a Knowledge domain cleric and ten levels of Lore Bard specifically. It seems like there’s not a hard and fast rule as to what you lose?
There is a table showing such things on the BG3 wiki ...
By going Cleric first you lose the Saving Throw Proficiencies of the Bard as well as some skills.
I understand that you are taking the Cleric up to lvl2 to obtain Knowledge of the Ages? It is nice

It is pretty much a wash, but I've found Rogue to Knowledge Cleric/Lore Bard to be best skill monkey.