Hi,
The CEAC status can sometimes be confusing because the text is written for all consular posts and each consulate has a slight variation on procedures. In our case, USC Montreal did not let us select our own visa appointment, but scheduled it for us automatically in the Packet 4 e-mail that they sent to us. This may have changed, but this is how it was 14 months ago.
For now, do not submit DS-160 - you need to wait until the embassy sends you a Packet 3 e-mail, which can takes weeks. Do not schedule your medical until instructed to do so in Packet 4. If I were you, I would get Packet 3 documents ready so that when you receive the Packet 3, you are ready to send it back checked and signed the same day.
If you are curious, the Packet 3 e-mail looks something like this (copied from my own records - 12/2021):
The list in Packet 3 includes a police certificate under the privacy act from RCMP, which is probably the most difficult document to obtain [see below - *]. You also need a police certificate for every country where you have resided for at least 6 months since the age of 16. See this State Department page for more details on required Canadian documents: https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country/Canada.html. In our case, we also needed to refer to the Brazilian documents page for birth certificates, military records, police reports. For example, if you studied abroad in France for 6 months, you will need to refer to the requirements for French documents.
The Packet 4 e-mail for K-1s looks something like this:
In our case, the website was not working properly so we were given an exception to pay the MRV fee in cash at the embassy -- it's probably been fixed since then, I hope! A PDF of the Packet 4 e-mail is needed to schedule the medical with the doctor via e-mail (see https://travel.state.gov/content/travel/en/us-visas/Supplements/Supplements_by_Post/MTL-Montreal.html for a list of doctors). During your medical exam, make sure to ask for DS-3025 (vaccine worksheet), which you will need for your Adjustment of Status after you get married in the US on your K-1 visa.
I hope this helps!
-D
[*] The standard police report, which is usually sufficient for Canadian purposes, is not sufficient for the US government because the report often omits details such as expungements, cleared arrests, and dismissed charges. If you visit a police station in your province, they will probably select "Reason: Immigration" in the fingerprinting computer, which will give you an incomplete police report that will be rejected by the consulate. Many officer are not familiar with the Privacy Act option, which gives you a full, unredacted report. We used Commissionaires (https://commissionaires.ca/en/) because their online system lets you select the Privacy Act option when booking an appointment (at least the Ontario division of Commissionaires does).