[1.21] How do I change the minimum height of sea level *and* raise the default 64 -> 128

As said in the title. I'm figuring out how to change the minimum sea level (which i figured out), the world height limits (also figured out) but I also wanna change the default "64" where stuff generates to "128". Currently that's my biggest trouble point. Ergo just raise the entire world by 64 blocks to give more space for cave gen. Been banging my head against the wall for the past 2 days trying to figure it out and I just can't. Would love to get some help <3