I definitely prefer [name_f]Thalia[/name_f]. I completely agree with [name_f]Talia[/name_f] seeming childish, and it feels incomplete to me. I pronounce both names as TA-lee-uh, although I’ve heard [name_f]Talia[/name_f] as ta-[name_u]LEE[/name_u]-uh and [name_f]Thalia[/name_f] as THA-lee-uh. I think in an English speaking area, TA-lee-uh will be the assumed pronunciation.
i’d assume both were “[name_u]TAL[/name_u] - ee - uh” (rhyming with [name_m]Hal[/name_m], not the word tall)
Sure! I’ve met two [name_f]Talia[/name_f]'s and they both spell it that way. I call them both “[name_u]Tal[/name_u]-e-uh,” but one of the girl’s sister calls her “[name_u]Tal[/name_u]-yuh,” so it’s interchangeable for her. The extra h in [name_f]Thalia[/name_f] looks wrong to me, not sure why. With names like [name_u]Theo[/name_u], [name_m]Thatcher[/name_m], [name_f]Thea[/name_f], etc. my first instinct is to call her “Th-al-e-uh” or “Th-al-yuh.” But then again [name_m]Thomas[/name_m] is “[name_m]Tom[/name_m]-as” with no problems. And also Teresa and Theresa with the h too. All my personal preference. Hopefully that helps 
[name_f]Thalia[/name_f] is way more formal and usable through a wider age range than [name_f]Talia[/name_f]. I live in Spain, and [name_f]Thalia[/name_f] would be pronounced ta-[name_u]LEE[/name_u]-ah, and ir sounds very pretty. No one has problems with that H in the middle, as in Spain we don’t pronounce that letter.
I feel that your personal choice would definately be [name_f]Thalia[/name_f] because the pronunciation won’t make much of a problem, as you mentioned, with names like [name_m]Thomas[/name_m] or [name_f]Theresa[/name_f] around.
I think both spellings are valid for your preferred pronounciation. I prefer the spelling [name_f]Thalia[/name_f] and would pronounce it with a hard T.