First is ensure all of your third party apps have been upgraded to 64 bit apps. I see you use MS Office for Mac, that will require Office for Mac 2016 or greater, however I would upgrade to Office 365 or Office for Mac 2019. Only 64 bit apps will run on Mac OS Big Sur which your new Mac will ship with and no, you cannot downgrade Mac OS.
To check which apps are 32 bit and which are 64 bit navigate to RoaringApps.com and research all the third party apps you want to keep. If you find they are 32 bit, they MUST be upgraded to 64 bit or 64 bit replacements must be found.
Also, when you get your new Mac please carefully follow the instructions below:
When you buy a new Mac the easiest, quickest and most reliable method is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes and follow the on-screen prompts and it's done! It is THAT simple.
If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts.
The ONLY difference between MA and SA is MA will create a new user account that you need to log into.
Both SA and MA migrate applications (not 32 bit apps) , data files, settings, it does not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not a big deal.
Finally, NEVER EVER use WiFi to migrate, this will fail in almost 100% of the situations in my experience. WiFi was never intended to move large amounts of data at one time.