That one spell that a DS gets from Affinity can be retrained for any Cleric Spell (but not Sorcerer). So by the time you get access to 9th level spells, trade your original Affinity spell for a 9th level spell, say True Resurrection.
Why can it be retrained for a cleric spell but not a sorcerer spell? I'm AFB ATM.
Btw if giving the Sorcerer more spells known is unbalanced then feats already do that.
It
could be unbalanced, if done without care for how much of an increase in power extra spells known gives.
For the spells known you get from feats, it costs a feat! A feat which could have instead given you something else.
For the spells you know because of your race, then the whole slew of abilities given by a race are already balanced against the abilities given by other races.
The Divine Soul sorcerer gets one more spell known, but that is their subclass ability, which is balanced against the abilities granted by other subclasses.
My comment about extra spells known
could be OP was in regard to subclass design. If an idea for a subclass used the format of two extra spells of levels 1 to 5, like the lists in the cleric domains, all as EXTRA known spells, then that subclass would need to be re-written because that would be extremely unbalanced, compared to other subclasses. To make it balanced, that list of 10 spells would need to be extra spells which they can choose from, while leaving their number of spells
known unchanged. Warlock patrons' spell lists work that way; they are balanced both because the number of spells known is unchanged, and each patron has their own list.
So when first noticing that Divine Soul sorcerers get both sorcerer AND cleric spell lists, you might get a moment of panic! You might have a knee-jerk urge to nerf it! Until you realise that the number of spells they know is unchanged by that extra list, while the subclass as a whole gives only one extra spell known, which is taken into account when balancing the whole subclass against other subclasses.
Ergo, this ability does not need to be nerfed.