It is impossible to reset screen time passcode

You are correct, you cannot reset the Screen Time passcode. All you can do is restore the iPhone to factory and set it up as new, not using a backup. The Screen Time passcode is part of the backup and will come right back. Follow these instructions to restore the device to factory. Restore your iPhone, iPad, or iPod to factory settings - Apple Support

You can see this support document with other information about Screen Time. Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

If you can't remember a restrictions passcode that you created on a previous version of iOS, or you forgot your Screen Time passcode, you need to erase your device, then set it up as new to remove the passcode. Restoring your device won't remove your old passcode.

This sounds like you may be using Keychain and that is moving the passcode back to the device since you mention the Mac. If you go to Settings>Passwords & Accounts on the iPhone, and you go to AutoFill Passwords, is that on? Is Keychain marked? You may need to turn that off. I don't see anything regarding the Mac in there, but I'm just starting to read this one, Use Screen Time on your Mac - Apple Support

Don't start a new iTunes account (Apple ID). Everything that you currently have purchased, downloaded, etc. is all tied to your current account. Did you read the information in the Mac link I provided? That says that as an administrator on the Mac you can reset it, unless you set up the restrictions in the administrator account. Are you in a family plan and was this setup by the Family Organizer? Otherwise, you may want to post in the Mac Catalina forum with the question about Screen Time following the devices.

I mean that you shouldn't setup Screen Time on the Mac while in Administrator login. When I upgraded to Catalina, it prompted me about Screen Time. While I don't have it set up on any of my devices, you had to make a choice there. I'm not as familiar yet with the workings of the Mac computer, I'm still learning. If you need more assistance with that, do as I recommended and post in the Catalina forum, or contact Apple Support for assistance. I cannot recommend how to handle the issue of restrictions on the Mac.

Yes, it seems that if you set the password on the administrator account, it will require that change. Read how to do it in the future if you intend to keep Screen Time active on your device.