Repeating "Reset Quicken Cloud" Message [Edited]

A couple of things to check:

Go to menu Quicken > Preferences > Mobile, Web & Alerts and make sure the Sync switch is OFF.

Quicken asks you to reset the cloud account when the file or a copy of the file has been accesses from another computer/user. Do you have this file stored somewhere that is synced to a cloud service, for example, Dropbox, iCloud Drive (Desktop or Documents folders), Google Drive, etc.? If so, that is not supported and could be causing the issue. Do you make a copy of your file and open it elsewhere, for example on your laptop when the file is used primarily on your desktop Mac. This can also cause this problem.

You indicate that Mobile Sync has no value to you. In that case, I would definitely leave it turned off. We often hear about the desktop data being corrupted/changed by Mobile Sync. Personally, I don't find that much value in Quicken Mobile or Web, so I leave it turned off to avoid these problems happening to me. The benefits of Mobile not worth the risk (to me).

Regarding Google Drive, yes absolutely do not store your file on Google Drive. This is likely the cause of your issue. Furthermore, there's a good chance your file permissions can be corrupted by this practice making the entire file unreadable. You can, however, safely store your backup files on Google Drive. 

If you need to access your Quicken file from two machines, the absolute safest way to do that is to keep the file locally on the primary machine. Then use remote access software to log into the primary machine to do Quicken work remotely.

Another alternative is to store a backup on Google Drive freshly created after using Quicken on one machine or the other. Then restore that backup to whichever machine you are using next. This process requires care to make sure you are using a current backup to restore at all times.

Regarding investment tracking: I use Fidelity as my primary brokerage and they still support detailed tracking. However, I don't really care. The reason is that I consider Fidelity's data to be "truth" and Quicken to simply be a copy of that data for reference and to be integrated into my overall financial picture. The portfolio and dashboard tabs serve that purpose just fine. But that's just me.