New Mac slow on Sonoma 14

To erase your drive:

Use Disk Utility to erase a Mac with Apple silicon - Apple Support

To start up in Recovery and install OS:

How to reinstall macOS - Apple Support

There is a note on reinstalling MacOS that says:

* If you just erased your startup disk, macOS Recovery might offer an earlier compatible version of macOS, but you can upgrade macOS after installation is complete.

Even though you are determined to rollback the OS, it may still be worth investigating the slow down you are experiencing by using EtreCheck as previously recommended. In addition reinstalling Sonoma after erasing your drive may also be worth trying to see if your experience improves. With your computer specs, Sonoma should not have any noticeable difference on the speed of your computer. I have a hard time recommending a rollback, but the information is there if you are determined to do that.

MacOS will not install any version OLDER that its currently-running version, unless you erase the Volume, which deletes all your files with no hope of recovery, and all traces of the newer MacOS. Then any appropriate version of MacOS can be installed instead.

¿what is the date of your most recent Trusted backup, and by what method?

You will need that backup to restore your files after ERASE.

Poisoning Internet performance:

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners. or a VPN that you installed yourself.

The idea that a third party, with no special knowledge of the inner workings of MacOS, can somehow find a simple way to protect your computer — that is not already being done by MacOS itself — suggests that the MacOS developers are somehow "holding out on you". That is absurd.

You should remove any and all (other than Apple built-in) virus scanners, speeder uppers, optimizers, cleaners, App deleters or VPN packages you installed yourself, or anything of that ilk.

Third-party file Sync-ers such as DropBox, BackBlaze, OneDrive, or GoogleDrive can ruin performance, but are not inherently dangerous.

Effective defenses against malware and ot… - Apple Community

To determine whether added software is the problem, run in Safe Mode, where no third-party items are loaded.

"Works in Safe Mode, Fails in regular mode" implies, "it's something you added."

Start up your Mac in safe mode - Apple Support

then the next step is to download and run they little 'discovery" utility, that looks around at your Mac from the inside and generates a report. save the report to the clipboard, and paste into a reply on the forums.

http://etrecheck.com

How to use the Add Text Feature When Post… - Apple Community

I think is should be this article:

Erase and reinstall macOS - Apple Support

it does not specifically say so, but I think you get a choice as to which version to re-install.

* If you just erased your startup disk, macOS Recovery might offer an earlier compatible version of macOS, but you can upgrade macOS after installation is complete. If the Mac logic board was just replaced during service, macOS Recovery might offer the latest macOS that is compatible with your Mac.