Burning Boot Loaders at Different Crystal Speeds

I’m new to the Arduino boot loader manipulation game and I have to set up the Arduino boot loader to 8, 12, 16, or 20MHz crystal (and/or any other crystal) and use 3.3 or 5.0V logic. What’s the easiest way to do this?

I have multiple UNOs, but they don’t seem to do boot loader shenanigans. I was looking at the AVR ISP MKII. Can that do everything I require? Can it not do what I require, or am I just way off my thinking, here?

Thanks!

If you get an ISP, you will not need a bootloader anymore --> no issue with different crystal frequencies as well.