You like to record the music she writes..
The music is played how?
How many different instruments at the same time?
Any focals while playing?
So basically :
What are you planning to do with it?
I think the thunderbolt interface is overrated?
USB devices are fast these days!
Imac can do your job easily...
I'm also running on imac with a Scarlett 18i20 with octo pre dynamic as extention and record rock/ pop bands and classic music on that..without any problems ( my biggest project : 170 channels )
I worked with presonus and I guess it is good software... I know some people would kill for it but it is not my thing.
Software is something very personal....
So please can you provide some additional information...
And finally as tip:
I don't know how much experience you have with recording but you mentioned you are just in the beginning of.....
Probably there are better products but better products doesn't make better recordings
My slogan :
It does not matter what you use,.... But how you use it....
You can make a million dollar record on a 8-track and destroy it on a 600.000 dollars mixing console...