I have axactly the same issue on MBP (13 inch, 2020). I would NOT upgrade if I were aware this issue. It is very annoying.

5 days ago, I posted my comment in this thread discussion because I had the same issue. Then, I have noticed that my extended monitor was trying to scan HDMI signal regularly and cause to flicker - I guess and believe that it can have an issue with adapter . So, I bought "USB C to HDMI Cable, CLDAY Type C to HDMI [Thunderbolt 3 Compatible], Supporting 4K with 6.6ft and Braided, Compatible with MacBook/Pro" So, right now, I am using USB C to HDMI Cable to connect directly my Macbook Pro with Dell monitor (24 inches), and it is working properly without flickering anymore - the issue went out. In short, you can try to use cable to connect directly your Macbook with your extended monitor - NOT to use adapter. Hope it can help and solve this issue. P/S: I am aware that this issue is really stubborn and annoying. Also, it is impacted to our work.