More than "partially", less than "completely"

"Largely"; "mostly"; "for the most part" ("...simplifies the problem at hand while for the most part retaining the features of...").

It's not quite what you asked for, but if you want to you might phrase it as "...simplifies the problem at hand while retaining all the relevant/vital/necessary features of..."

predominantly fits into the spectrum snugly between partially and completely.

The use of 'partially' has the 'cheap copy' effect, and focuses the reader's attention on what was not included.

the model in question has several nice features and simplifies the problem at hand whilst partially retaining the important characteristics of the original system.

The model in question has numerous excellent features and simplifies the germane issues whilst consciously retaining the important characteristics of the original system