Screen Flickers after MacOS Catalina Update

I don't think that would do any good. Try starting your mac while holding down the Option key. You should get a screen allowing you to choose a disk to start from. Is that screen flickering?

Note: at that point, no OS has been loaded yet, so if it is flickering, it is most likely an issue with the cable connecting the motherboard to the display. The good news is that this is an easy fix, but it has to be done by an authorized repair center.

Download Etrecheck (free) from the App Store.

Run the application to let it produce a report on your system.

When finished, click the Share Report button and choose Copy Report.

Open a new message here, then click the "additional text" button below and paste the report.

As you can check, the report carries no personal information. It is meant to help us help you by giving information about your hardware and software.

Does the flicker appear right from the start after you turn your mac on?

There must something specific to your mac.

I see that you already tried booting in Safe Mode, and it seems that the problem also occurs.

Try resetting NVRAM and SMC. Any changes?

What mac is this? Do you have a chance to connect it to an external display? If you do, tell us if there is also the same flicker on the external.

For more specific advice we need to have more information. Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.