First, PC with FDE + normal boot gets stolen. The attacker cannot get the data without the password, so it's safe.
Second, unattended FDE + normal boot PC gets tampered with. Attacker manipulates the bootloader. Unsuspecting user later boots the tampered PC, unlocks the FDE, gets owned.
The second case requires a professional, dedicated attacker. I use TPM with Heads and a hardware key to protect myself against it. It will notify me if the boot partition or BIOS are tampered with.
As an advantage, all relevant code running on my computer is FLOSS and auditable, unlike the Secure Boot and UEFI.
And yes, getting back to the original topic, I believe that against petty criminals, even a full disk encryption is plenty defense. They won't go about installing anything to the EFI partition just to get to the data.
This Coreboot + Heads setup I'd trust to protect against even the more involved.
Unless your BIOS/UEFI supports full disk encryption unlocking of hardware-encrypted Opal drives, you will always have an unencrypted bootstrap process at early boot from the disk.
That unencrypted bootstrap process can be modified by anyone with access to the disk, physical or remote. Theoretically, someone can inject a keylogger into the process and exfiltrate your encryption key's password, or a process that waits until you're decrypted and exfiltrates your data. It's also a potential vector for ransomware, root/boot kits, etc.
What you've described in that comment is just kind of reinventing the wheel. You're solving the same problem a different way, in a way that has slightly more complexity than just using UEFI and secureboot.
The main difference is that the user owns the root of trust and doesn't have to blindly trust a (buggy) proprietary software from a commercial company. Also, the community review.
https://security.stackexchange.com/questions/267222/full-dis...
So, there are two scenarios here.
First, PC with FDE + normal boot gets stolen. The attacker cannot get the data without the password, so it's safe.
Second, unattended FDE + normal boot PC gets tampered with. Attacker manipulates the bootloader. Unsuspecting user later boots the tampered PC, unlocks the FDE, gets owned.