@orienteer One approach to advocate for yourself is to be aware of the words used in this upcoming mammogram report. Sometimes they will wait until after the mammogram to decide if they need more scans (MRI or US).
Below are some things to look for on your report. If you find that there might be an indication that the DCIS has grown or changed shape in the report - then they should be able to tell you the plan to address the changes, and maybe ask for an explanation of why they would not do a MRI, and what is the criteria they use to decide for/against an MRI.

Ask what letter designation has been given for your breast density it may give you more info to use to request an MRI. A mammogram usually is used to determine density - but it doesn’t always show up in your report as A-B-C-D, they usually just say dense or not dense. The A level is almost entirely fat cell, D is highly dense. I was at C level at diagnosis and had a MRI. I’m now at B because tamoxifen has an effect on density at my age. But I’m still listed as have dense breasts, and have had yearly MRIs since surgery for DCIS 0, 1.6 cm, intermediate grade, one sister with DCIS.

A mammogram report will use specific terminology to describe the morphology (shape) and distribution of calcifications.
Suspicious words detailing changes, growth, or increasing calcifications include:
Morphology: Words like pleomorphic (varying in size, shape, and density), linear, or amorphous (small, hazy, and irregular).
Distribution: Terms indicating the spread, such as segmental (within a lobe/duct system) or linear (arrayed in a line suggesting a ductal deposit).
Progression/Changes: Look for phrases like new cluster, interval increase in extent, increased number of calcifications, or change in morphology.

Expected DCIS Growth
What is it? DCIS is stage 0. Growth generally happens linearly, constrained within the milk ducts, rather than forming traditional, round, expanding tumor masses.

Intermediate Grade: The cells look more abnormal and grow slightly faster than low-grade DCIS. Calcification studies show this grade extends along the ducts at an average axis of about 4.2 mm/year.

High Grade: These cells grow the fastest and look the most abnormal under a microscope. Calcifications for high-grade DCIS can spread along the ducts at a faster rate of roughly 7.1 mm/year.

With these numbers, a six month growth might be half as much, but I don’t have facts about that. If you were at Low Grade, and there is new growth you can look at the speed its grown in case it has changed to a higher grade.

A biopsy using ultrasound is very typical. I have found through reading or listening to the doctor, that a lesion 2 cm in size triggers a much more thorough exam. If its smaller, then there is usually other factors that push them to do more extensive scans. That might be, high grade or intermediate grade that has features they are concerned about, sometimes its the location in the breast or family history, etc.

What I’d do is: when you get the latest mammogram report check for changes in the dcis. If you have a way to message the doctor in your health portal, I feel that if I put my concerns in writing they pay more attention as it becomes part of my medical record.
Besides the growth, I would ask about if it says amorphous, or segmental. A change to amorphous is probably a morphology change and more scanning might be needed. Segmental might mean its grown toward a lobe, and more scanning might be needed, although I think a mammogram is supposed to do a better job of identifying LCIS.