Vividblu99 Firstly I will say that compared to a lot of people on this forum I do not appear to suffer with as wide ranging or impactful symptoms when looking at a screen. Simply intense eye strain / tiredness and nausea. Naturally the strength of this varies and also the length of time it takes my eyes to recover from hours until the following day.
I was aware the Poco M5 was no powerhouse however I found the phone slow and software buggy upon initial setup with prompts appearing on screen in relation to this. I factory reset it and tried again however still was not happy and coupled with some strong eye strain I decided even if my eyes got used to the screen I wasn't happy with the initial experience of the software and hardware.
I genuinely do not know what the issue is with screens that causes my symptoms. I am fairly confident it is not PWM as I have had issues with devices that do not have PWM. I also have issues with LCD screens so do not take too much notice of the LCD vs OLED debate.
In the past I have returned the iphone SE (2016) and Sony Xperia Z3 Compact (2014) amongst other LCD handsets. I have been using a Google Pixel 3a for around 4 years now which is OLED. I recently looked at a friends iphone 15 Pro Max (OLED) and it didn't appear to cause any issues but that device is way out of my price range.
The other challenge for me is I prefer small handsets so around 6 to 6.2 inches so options are limited however if I found a 6.5 incher that I could use then I would still be very happy. I have just bought a refurbed iphone XR from Ebay as well as a screen protector for the first time to see if it helps though I doubt it, I know you have had success with screen protectors on some devices.