Althea is nice. I slightly prefer
Anthea or even the similar
Athena, but I'd dearly love to meet someone named
Althea! I agree with you that
Althea has nice nn options. BTW,
Thea is one of my favorites (actually,
Thea Madeleine).
Re: your combos, the mns are all nice (well, except for Bodhi), but they contain a lot of syllables. If it were me, I'd use something shorter:
Althea ClaireAlthea CorinneAlthea DaphneAlthea FrancesAlthea HelenAlthea JaneAlthea JoyAlthea LouiseAlthea Madeleine (an exception here)
Althea Marielle (ditto)
Althea MaryAlthea SophieAlthea Zoe
I hope you get to use
Althea. It's a lovely and unexpected name.