[name_m]Hi[/name_m] there.
[name_f]Lottie[/name_f] [name_u]Wren[/name_u] is sweet. I think that it’s totally fine to do just [name_f]Lottie[/name_f] if you love just [name_f]Lottie[/name_f]. It’s not too cutesy. I only know one adult [name_f]Charlotte[/name_f] who goes exclusively by [name_f]Lottie[/name_f]. Feels like it grows up just fine.
[name_f]Hope[/name_f] this helps. 
I think [name_f]Lottie[/name_f] is fine as a standalone name and [name_f]Lottie[/name_f] [name_u]Wren[/name_u] is gorgeous!
Lots of names that no-one would question began as nicknames eg [name_f]Sally[/name_f], [name_f]Sadie[/name_f], [name_f]Pippa[/name_f], [name_f]Wendy[/name_f].
And even it it is a nickname (in origins) it’s completely fine to choose to use it as the full first name. I’m guessing that you’re in the US. In UK or Australia it’s much more common to choose a name like [name_f]Lottie[/name_f], [name_f]Maggie[/name_f], [name_f]Maisie[/name_f] etc as the name.
I know a girl just named Lottie (after her moth, whose name is Charlotte!) — I think it’s very cute. I also know an adult Charlotte who only goes by Lottie!