Android Messages backup & restore

Hi, I have a Google Pixel 2 XL and I have used the Android Messages app for all my SMS/MMS messages since I first got the phone. I currently have the latest version of the app, which is 3.3.044. I am planning to factory reset my phone, but before I do so, I'm trying to ensure that I will be able to restore all of my SMS/MMS conversations afterwards. Some of the messages are extremely important to me and I'm not willing to risk losing them. How can I ensure that I get all my conversations back intact, with both SMS and MMS included, just as they appear on my phone now? I know that Pixel 2 backs up my SMS messages to Google Drive, however it specifically states that this does not include MMS messages, which poses a problem for me. I need my conversations to remain intact, in order, exactly the way they are now, with both SMS and MMS messages included. I was excited when I learned of Android Messages for Web, and thought this may solve my problem. I noticed every single conversation appears on Messages for Web exactly as it does on my phone, including both SMS & MMS. This initially led me to believe that Messages for Web has stored a backup all of my SMS/MMS conversations online, but then I came across where it says 'Stay connected to your phone. Messages for Web connects to your phone to sync messages' on Messages for Web. Does this mean that if I factory reset and delete everything on my phone, a backup will not be stored on the Android Messages for Web? How can I ensure that I retain all my SMS/MMS conversations intact upon factory resetting my phone and reinstalling the Android Messages App? Thank you very much in advance for your help, I greatly appreciate it.