FYI, You may or may not get quick responses when posting on a forum. If you want instant support, then you need to contact Apple support directly where you can have an online chat with them.
As for the Apple Diagnostics, I would be surprised if it reports any problems. The Apple Diagnostics....even the Apple service diagnostics don't actually do much testing these days. They basically just check to see if the hardware is present & has basic communication. Even when there is an actual test, it is just a very quick & basic test.
If you just purchased this laptop, then I highly recommend you return it for a full refund. This specific model laptop has an extremely high rate of Logic Board failures (T2 security chip, SSD, memory, GPU).
Plus it sounds like you may have more than one issue with this laptop, or perhaps it had liquid damage which can explain its multiple problems. It doesn't sound like a refurbished laptop, it sounds like a worn out & broken used laptop someone wanted to unload. A lot of unscrupulous sellers will just say "refurbished" where at best they may have reinstalled macOS.
If you want a Mac, then I would suggest avoiding any of the USB-C Intel Macs for multiple reasons. Get an M-series Mac instead, but be careful buying used since there are many unscrupulous sellers and a lot of innocent sellers who don't properly prepare a Mac for sale & you may find that someone else is still managing that used Mac. Buyer beware.
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
If you want to buy a properly refurbished Mac, then check out the official Apple refurbished options.....they will vary depending on what has been traded in by users:
Refurbished Mac Deals - Apple
You can also check out OWC's refurbished Macs (OWC has been a respected vendor supporting Apple products for decades):
https://eshop.macsales.com/shop/Apple_Systems/Used/Macs_and_Tablets
