WoodenMarker

Are the standoffs fully tightened? I’m not sure if they use the same threads on each end, but check to see if one end is coarse and the other is fine.

Try updating your BIOS to the latest. Your current version is very old.

The original shutdown may have damaged the motherboard. Replacing the motherboard is a reasonable next step. Is the motherboard under warranty? If so, submitting an RMA for a motherboard replacement may be the cheapest solution. For an RMA description, something along the lines of:

You can install the heatsink after installing the cooler mounting hardware, motherboard, and power cables.

It may be that the default BIOS settings may have reset boot or display related settings. Try recovering the BIOS from the original hard drive. https://www.dell.com/support/kbdoc/en-us/000132453/how-to-recover-the-bios-on-a-dell-computer-or-tablet If that doesn't work, there's a method 2 for flashing the BIOS with the same recovery keys.

Try the RTC reset to see if that works. https://www.dell.com/support/kbdoc/en-us/000124377/how-to-perform-a-bios-or-cmos-reset-and-clear-the-nvram-on-dell-systems

Although unlikely, the gpu may be the cause. You can rule this out by removing the gpu and seeing if symptoms are the same. A grounding issue with the case is also a possibility. You can rule this out by testing the motherboard, cpu, ram, and psu outside of the case and turn it on by shorting the pwr_sw pins. If the issue was not the psu, the next most likely culprit is the motherboard. Other failed parts are also possible but less likely.

Was the machine working fine before? Have you tried reseating the display cable and making sure the correct display output is being used on the monitor? Is the monitor plugged into the back of the graphics card?

Just the cable from the psu should work fine.

No, each header on the motherboard is labeled and the corresponding settings for each header in the bios. Some header settings like CPU_FAN and CPU_OPT are often shared. If you want to connect fans to different headers, fan splitters can be used.

If going back to the 2060 works fine, it's likely a faulty graphics card and you should contact the seller for a replacement.

It may be a faulty graphics card. Are you able to test the 5070 Ti in another known working machine?

No. RAM can attempt to run with whatever settings are set. Advertised speeds and EXPO profiles are manufacturer validated settings that are expected to be stable. The motherboard defaults to JEDEC speeds. The advertised RAM speeds are technically an overclock and selecting either of the profiles loads those settings. Try updating the BIOS and see if that allows you to select the EXPO profile.

Seems like it failed to POST and succeeded on retry. This is usually due to memory training and can happen occasionally. There's no need to worry about it if only happens once in a while.

You'd lose up to ~5% performance going down to pcie 3.0 x8 for the 3080. A secondary gpu can increase performance with LSFG. Are you aiming for high refresh rates and are unable to hit them with the 3080 alone? A 3080 should be already plenty for 1080p.