I have a way that I think it's working acceptable. I will say, the problem is not solved for 144Hz, but for me this is acceptable:
I created a new resolution of 3440x1440 by 141Hz (140Hz gave me strange artifacts on the OSD), and 142Hz or 143Hz was displayed on the OSD as 144Hz and the flickering still occurd. But when you set it to 141Hz, the flickering is gone. I still have FreeSync and G-Sync enabled.
Also someone on the Dutch forum Tweakers wrote that his issues where gone when he put it to 146Hz. Unfortunately for me this causes enormous frame skipping. You can test frame skipping here:
https://www.testufo.com/frameskipping
You need to set your (smartphone) camera to take photos with an exposure of 1/10. You need to have a straight line of frames without any skips. When I put my screen to 146Hz it looks like this:
As you can see a lot of skipping frames. At 144Hz (and 141Hz), it looks like this:
To make it easy for you I exported my CRU config. I've includes both 141Hz and 146Hz so you can test the setting you'd like. CRU file download:
https://gofile.io/d/Oa2CIh
You can change these setting yourself by doing this:
Open CRU, in the "Extension blocks" part of the screen click "DisplayID" (if not exist, create it), click add, click add now create new resolutions of 3440x1440 by 141Hz and 146Hz.
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