Learn English – More than “partially”, less than “completely”

I am re-phrasing several sentences in a manuscript we are preparing for publication and I have come across a sentence I am not quite happy with.

Without going into the details, the sentence is about a model system where we claim that 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 take home messages here are:

1. The model is simple enough that we can work with it
2. The model does not reflect the reality entirely, and has some issues associated with the compromises made in order to simplify the problems in the original system.
3. However, the model is still relevant enough that it's worth working on it

My issue with the word "partly" or "partially" is that it does not convey right level of conformity between the model and the original system. To my ears partly/partially sounds like there is some overlap between the models, but not necessarily enough to justify the amount of time and money invested on it.

So my question is; is there a word that I could use in this scenario, one that is more than partially but less than completely?

Follow-up question; when it comes to adjectives and adverbs where one can rank them, is there a good resource to check on, when one is in doubt? I usually use Thesaurus to find words that are related to one another, but I have not found one that actually ranks such words.