I think the epithet about making a bit of their own champagne on the side only applies to some of them. The rest probably use all their grapes to make their own champagne, or just sell their surplus grapes. It varies across the whole spectrum of business practice.
If this is an area you have not explored before, I think I should warn you that a lifetime’s obsession may await you! Be warned.
Some points may be helpful before starting. There is a tremendous range of styles from different growers, and sometimes that can be true within their ranges as well as from grower to grower. Even allowing for sorting the wheat from the chaff, there will be some styles that appeal more than others. Just in the same way as there are differences between the big houses of course.
A good book to get hold of is Michael Edwards’ “The Finest Wines of Champagne” - he covers a good slew of growers as well as the important big houses. Of course he leaves plenty out that are good, and it’s a few years old now, but by no means is it out of date.
By the way, the RM designation reliably indicates a grower’s own champagne, but there are genuine growers registered as NM (=Negotiants-Manipulants), the same as the big houses. The reason is often to do with French inheritance laws - one member of the family may inherit their fraction of the family estate, but to produce a balanced wine they may need to swap or buy in a proportion of grapes, often from family members owning another part of the original estate.