You can grab the kernel source and run make localmodconfig to generate a kernel config based on the currently loaded modules.

Then in nixos you can override the kernel config with that one (sorry, I can't get the exact instructions right now).

You'll need to make manual config changes for anything you might need to load dynamically.

I’ve heard that the original Linux kernel binaries+blobs are pretty huge

The kernel is <10MB and the initrd <20MB. "Pretty huge" is relative here.

You could override your kernel and strip stuff out of the initrd and I can assure you that it's going to be a lot of pain but it's easier to just make a 1GB boot partition these days and pretty much never worry about bloat again.