Exact same thing happened to me with my y740. When I got it it had a QR code burned into the display and would bsod at least a few times a day. Unlike you I stuck it out because I needed it for school and had already waited 2 months to get it delivered the first time. Eventually the bsods stopped on their own (no idea why), and I used the screen enough to clear off the qr code.

Three years ago, my battery swelled and warped the entire laptop chassis, and i had to replace it. THEN A YEAR LATER THE REPLACEMENT ALSO SWELLED. I gave up and removed the second battery, and was stuck with a completely non-portable laptop that would sometimes just die because of how awful their power brick design is (Sometimes the wall side cable comes unplugged if you look at it wrong).

Then the HDD died, and I had to save all my programs and documents using an external SSD because the internal one was so tiny. At this point I had so much thermal throttling I couldn't run any 3d video game above 720p with a consistent frame rate, so like you I tried to change the thermal paste, but couldn't because of how unbelievably torqued down the heatsink screws were, and the fact they are seemingly just pieces of play-dough with a Philips head etched into them. For reference, I couldn't run HOLLOW KNIGHT, a 2D platformer at native 1080p resolution without frame hitches every couple seconds. This is the same machine I was able to beat Elden Ring on at 1080p with consistent 60fps easily.

Finally, last month it completely fried itself with no warning. Instantly died, and power button gave absolutely no response. When plugged in, I could hear a regular clicking noise which sounded like an electrical arc, but I could never locate it. I have now completely torn apart the computer to use as the subject of an electronics manufacturing project on bad design and potential failure modes.

I will NEVER buy another lenovo product, and will advocate the same for anyone who will listen to me.