I think it is best to avoid the th because of the lisp. If it were you with the lisp, I think you would want to avoid it too.
How about Elsa, I think it is a beautiful name, simpler than Elspeth but obviously easier to say and also just as lovely (IMHO). It means God's Promise or God is my oath.
Similar names are: Ellsa, Elisa and Elysa.
I love the sound of the name Tacita. It means silent or to be silent, which is why I am not so keen on it as a name for a girl! One website even used the word 'mute' as a meaning for it! Having said this, there are lots of other names (like Gilly) that can mean quiet, so I would not get hung up on it if you like the sound of it!
Similar names might be Taitianna, Taegen, Tatijana, Taygan (English), Taziana (Italian), Teagan (English and Irish), Teegan (English), Tegan (English and Welsh) and Teghan (Welsh). Not sure if/how the lisp would affect the pronunciation of any of these.
If you go to you can see links to all the pronunciations!
Yes, I am getting these from a website, I am not a dictionary of names!
Last one, I love but I know DH would not and also we are struggling big time to have another baby - Cadence meaning rhythm or with rhythm, also spelled Kadence.
Good luck and congratulations.