With regard to mechanics, if you want to design a new lineage, use the Custom Lineage in Tashas.
Then create your own feat for that lineage.
An easy way to do this, is to look at the half feats in Xanathars and in Tashas. These are the feats that also grant a +1 ability score improvement, but ignore this improvement. Create two half feats for your lineage.
For one of the half feats, to cast one 2nd level spell per short rest, seems balanced (compare Fey Step), albeit this is probably the most powerful edge within the boundary of balanced. Other feats have a 2nd level spell per long rest, but then add extras. It also depends on which spell; some are more powerful than others.
Where Tashas gives the option to have Darkvision or swap it for a proficiency, I feel strongly it is balanced to swap Darkvision for a cantrip, such as the Light cantrip.
When I design a lineage feat, I tend to pack it full till its bursting at its seems. But that is ok. Because, the Tashas lineage is less powerful compared to the lineages in the Players Handbook. So as long as one is aiming for the Tashas design space, the end result is highly likely to be balanced for the game.