It is perfectly possible to use iCloud to sync the Mac Mail configuration, including mail rules, across multiple Macs. However, there are some gotchas!
The first thing to note is that you must do things in the right order - if you don’t you risk losing all the rules you have so lovingly created! I’ll deal with that later.
The second thing is that you need to be aware of the things that make up the Mail environment. For example, your rules may refer to entries in your Contacts - and so you need to ensure that your contacts are also shared in iCloud (on both machines), otherwise the rules may behave differently according to which machine they are run on.
If you use any Mail addons, you will need to ensure that these are also installed on both Macs.
In essence, you need to ensure that the mail environment is identical on both machines, meaning that everything relevant to Mail must be in iCloud, otherwise things will mysteriously fail to work!
As an important part of this, you need to look at the mailboxes used. If you look at the mailboxes panel in Mail (the left-hand of the screen), you will find that mailboxes are grouped into Favourites, Smart Mailboxes, iCloud, On My Mac, and then by mail account. In order to successfully share your mail setup across machines, you must move all mailboxes under “On My Mac” to “iCloud”. You can do this by simply selecting them all, and dragging them to the “iCloud” heading. Note that the iCloud heading will not appear unless iCloud Drive has been “turned on”.
All the relevant bits of iCloud are controlled from System Preferences.
(The following applies to Big Sur - the instructions may be a little different for earlier versions of MacOS.) Open System Preferences, and click the Apple icon (“Apple ID”). On the left hand side of the panel that appears, click “iCloud”. You are now looking at the crucial control pane.
If “iCloud Drive” is checked, then it is “turned on”. If “Mail” is checked then Mac Mail is saving its settings in iCloud.
Let’s define some terminology. The pane referred to above is “the control pane”. You have an existing Mac, currently running Mac Mail, on which you have a setup including Mail Rules. I will refer to this as Mac A. You also have another Mac, on which you wish the share the setup from Mac A; I will refer to this as Mac B.
Step 1. For EVERY Mac registered to your Apple ID (including Macs A and B), ensure that Mac Mail is not running (quit out of it if it is), and that on the control pane “Mail” is UNchecked.
Step 2. On Mac A, open the Mail app, then go to the control pane, and ensure that “iCloud Drive” is checked, then check “Mail” and “Contacts”.
Step 3. On Mac A, in the Mail app, move all “On My Mac” mailboxes to “iCloud”, then quit the Mail App, and re-open it. (This ensures that all relevant data is flushed to iCloud, and mailbox files are moved to iCloud Drive, so there may be a pause while mail quits.)
Step 4. On Mac B, open the Mail app and install any Mail addons that are used on Mac A, then quit out of Mail. Go to the control pane an ensure that “iCloud Drive” is checked, and check “Mail”. Now open the Mail app - you should be sharing the Mail environment of Mac A.
Note that it may take several minutes for everything to propagate across from Mac A to Mac B, so after starting Mail it may be best to go away for half an hour before trying to actually do anything in Mail. When you come back, you should find that all your rules, smart mailboxes, other mailboxes, and everything else about the Mail environment for Mac A are all in place! on Mac B!
A word of caution: the above is, to the best of my recollection, what I did to get my own mail environment running on two Macs, but I may have misremembered - I take no responsibility for any mistakes or omissions!!! I would therefore most strongly advise that you ensure you have up-to-date backups of both your Macs before you start! Good luck! 