I also love Tahlia- and agree that Tahli/Tahlie is a sweet alternative. Otherwise, you could perhaps consider Tahlia Anne, instead of Anna? That way you are still using the name, but a slightly modified form.
I just don't think it is worth compromising on a first name you both like, for the sake of a middle name. They only appear on legal documents, afterall, and are hardly verbalised in everyday life.
Otherwise, some suggestions for you;

Arielle Anna (If you don't mind 2 'a' names)
Shanelle Anna
Jenae Anna
Jacelyn Anna (Jay-se-lyn)
Mahlie Anna (Like Tahlie!)
Madeley Anna
Shaylie Anna
Shiane Anna
Tamsyn/Tamsin Anna
Tamlyn Anna
Taelyn Anna

I threw a few T names in there incase you like them.....

I have such a huge girls name list, but I didn't realise how many ended with an 'ay' or 'uh' sound until I did this post!