First off I don't recommend implementing the specification from scratch. It is big, and there are implementations already in various languages: for C you can use GNU-UEFI, for example, Rust has uefi-rs, and Zig has support in its standard library. These take care of the fiddly details of interacting with the UEFI firmware's services.

There's tutorial type articles for these around: for C there's https://wiki.osdev.org/UEFI_App_Bare_Bones, for Rust there's this template https://github.com/rust-osdev/uefi-rs/tree/main/template and the crate's documentation.

I don't think the role of firmware requires it to be minimal. It's to provide a consistent boot environment on different hardware, and to provide useful services to the program. Even BIOS-based machines have significant amounts of code that run before they jump to the boot sector, so while UEFI is definitely more sophisticated I don't really see this as a drawback to it.