The 9750H is a mobile processor, meant to be soldered to a laptop motherboard. It will not fit in a a desktop motherboard, nor will you be able to buy one. So for a desktop you don't have a choice. Unless you buy a notebook.
That said, why would you buy the 9700K now? According to benchmarks the cheaper Ryzen 7 3700X is slightly slower in some tasks (single ...
The Ball Grid Array used in the Core i5-8350U CPU of the Thinkpad T480 is factory-soldered, and I would not even try to remove it to upgrade the CPU for any one of three choices:
1) Unsoldering and resoldering BGA CPUS is very difficult without factory-level tools
2) The chipset, also permanently soldered onto the motherboard, limits your CPU choices.
Since the Acer Zenbook Duo only has DDR3 RAM, and the inferior MX250 GPU, I'd rule that out.
Your PH315-52-76WX model of the Acer Predator Helios 300 looks like a good buy, but the Model B07S8D3C8J TUF Gaming A15 with 256 GB M.2 NVMe SSD, Ryzen 7, and 1660Ti GPU has a faster internal drive and WiFi 6, whereas the Predator only has WiFi 5. Of course, if ...
Most 4GB laptops can be easily upgraded to 8GB, which might be enough for you if you prefer cheap solutions.
"Gaming Laptop", "office laptop" and the others are mostly bullshit-words to help laymans to choose the product.
AMD is the same in benchmarks like the intel, but it is cheaper. They need to compensate their weaker name with price.
In your case I ...
It isn't going to fit.
The LGA 775 socket CPUs are a different form factor, 37.5mm square, than the i5-8400 which uses an LGA 1151 socket.
The distance between the screw-holes for the heatsink of an LGA 775 is 72 mm. Such heatsinks are not interchangeable with heatsinks for sockets that have a distance of 75 mm, such as the i5-8400 which uses the LGA 1151 ...
Check that the monitor power cable and HDMI cable are securely plugged in. If the issue still occurs the cable is probably bad, replace it and see if that works. If that doesn't fix it, either the HDMI port on the monitor is bad, or the HDMI port is bad in the GPU. The ports shouldn't have gone bad after a maximum of a month of usage though.
I currently have a Ryzen 5 1600 + GTX 1080 setup. I'm getting about 100fps in most games at 1440p165hz.
So if you really want to push 144fps, I would recommend getting a 1080ti or better.
RTX 2080 (Super)
RTX 2080ti (Super)
You might be able to get away with a 2070 (Super), but I would get one of the above.
It is perfectly possible to do development on a laptop with 8 GiB of RAM - I've done that for years on end now. However, you must assume that you will be working within an IDE such as Visual Studio. These kind of applications thrive on screen space. A laptop often has too little of that, so it is much better to work on a larger, secondary screen. Probably ...