While you acquired the machine in 2018, it is a very old design (Intel Gen 7 arrived in 2011) so not all the parts may be there. Is there a date on the BIOS setup menu?
As was made abundantly clear above.
TPM 2.0 debuted in 2019 and was likely quickly taken up by companies like Dell. It isn't something you can retrofit. That said, your old machine doesn't qualify for an "upgrade". You can install Windows from scratch but I doubt that's what you want to do.
Not if it is working correctly. UEFI boostraps from the boot drive rather than from the BIOS. In most modern computers, UEFI is much faster to boot.
My Intel Gen 4 Debian 11 box boots to the desktop in about 18 seconds (depending on how quickly I enter my credentials). Under legacy mode it took about three seconds longer.
I expect that what you're experiencing may be a result of Windows not having been originally installed with UEFI.
I'd be happy that nothing is going to break on the old machine as a result of Microsoft's seemingly random changes that result from their incomplete effort to make all of Windows look like Windows.