There are NO viruses for Mac at this time. Not through any vector. Nor is any vector for any future virus known at this time. Basically, macOS is pretty secure. Anything that installs to your machine has to have YOUR permission to install.
They COULD potentially, if they had your password, try to come in remotely and have your system reboot, but on that reboot, they would have to have direct access to your keyboard to enter the password or select the recovery partition to boot because in the boot process you have to have a keyboard entry before the network connects. But I told you to change your password. I said that because even if they have your current password, they won't have the new and therefore won't be able to do even that reboot. The reason I said "potentially" is that if you ask a question about "Can something be done?" I have to say "yes, anything is possible." Pigs could fly, all the air in the room could decide to exit through the window and leave a vacuum, you could flip a coin and have it land on the edge, you could be hit by a meteorite tomorrow, etc. But the probabilities of all of those is really, really small and not worth worrying about. But NOTHING is guaranteed in this world, ever. Anything can happen.
And that's as objective as it's going to get. At this point you sound like you are never going to trust anything, ever again, which is kind of sad. If you are that afraid of this "internet stalker" then don't go online. Seriously, there is a life without the internet, and it sounds like in your case it would be a better life.
You have a very secure computer in the Mac, you say you use VPN, you have reinstalled the OS and you can (and should) change your passwords. That's about all any person can do to keep a bad actor out of your system. Don't let the stalker win by creating such paranoia.
I used to work in the highly classified computer industry where our data was super-classified and sensitive beyond belief. Our assumption in that industry was that there was NEVER a truly secure system. Two people cannot keep a secret. And computers are always going to be able to be cracked. The closest to a secure system I ever heard about was a CIA system that had double filtered power supply to a computer in a vault with total electronic shielding, no printer, one terminal in a locked vault with two guards on it 24/7. And nothing went in or out of the room except cleared agents. Basically the operator went into the room, took nothing in, did the query on the terminal, got the answer, memorized it, and walked out, taking nothing out. Even their pockets were emptied and checked by the guards. We thought it COULD be secure, but we could not guarantee it. Because as I said, NOTHING is guaranteed. So you can see why my mind runs to could/should/ought to instead of will/does. Don't let my paranoia and hair-splitting about "can something be done" get to you. I already drive my wife crazy, but after 40 years of being in that industry, it's just how my mind works.
If you want to go one more step, you could encrypt your hard drive. The risk in doing that is that if you ever forget that password, there is NO way to recover it, or the data on the encrypted drive. Given your paranoia, what I would suggest if you decide to do this is to take the Mac offline totally (No network, no WiFi, no BT), make a backup, test the backup, and then open System Preferences, Security & Privacy, click on the FileFault button and turn on FileVault. It will ask for you to create a password and then will create a recovery key for you. Then let it run to encrypt your hard drive. It will take a long time to complete, so let it run until it is done. If you interrupt it, you will end up with a totally unstable drive that probably won't boot any more. You will have to recover from the backup or from the internet and format the drive to recover it and repeat the process. So DON'T INTERRUPT IT. (Yes, I am shouting.) Of course, if you are still convinced that this bad actor can see every keystroke (which I seriously doubt, given what you are doing), then you may work that on the entry of that FV password this stalker will have it, too. But probably not.
I say that because with FV turned on, when your system boots you will have to provide the encryption password before it even starts to boot, and then you will have to enter your account password after it boots. But any "intruder" to your system will be totally stymied by the FileVault encryption because the FV password comes well before any network connection is even opened, so even if a bad actor could somehow force a reboot from outside, the FV password will keep it from booting. Once you know it's done encrypting, you can turn on the network (WiFI and BT) again. Oh, and if you make backups of this encrypted system, make sure to encrypt them, too. You can do that in most backup software, including TimeMachine.
You can read more about FileVault here:
https://support.apple.com/en-us/HT204837