It is often credited to Einstein that the "definition of insanity is doing the same thing over and over and expecting different results."
If flashing again and again doesn't work, then maybe perhaps try changing some advanced wireless settings like Coverage Cell Density and Distance Optimization. I have set mine to Normal and 100 respectively. Search the net for topics regarding those two.
I have been running my custom builds of OpenWrt 22.03.3 across multiple devices sharing almost the same cpu and wifi as Newifi 3 D2 (MT7621AT, MT7603EN 802.11bgn, MT7612EN 802.11nac) like the R281/R291, S12 Pro, and FX-ID4, and I did not experience such wifi drops among them. Granted I don't currently own nor have I used a D2 from before and I had to do a workaround in some of the devices to get better txpower settings, but you should try the above to see if it improves in yours.