How much do you think is enough, and why do you think the amount collected during boot is insufficient?

Let's say 300 bits.

I'm assuming a bootloader can't collect more in three seconds than an OS can collect in multiple minutes. Is that assumption wrong?

If it's the design of the code that lets it collect entropy faster, then it's that code that needs to be ported, and it doesn't matter if it's in a bootloader or not except to save a second or two.

300 true random bits is a lot. Even something like 128 bits is unbreakable with available hardware.