As of 1.20, both of these combined would allow for a skyblock start without a tree - instead, starting with only some bamboo in your inventory!
I can imagine bamboo not infesting on the bottom block, and having a low (maybe 20%) chance of dropping string.

Fair enough!

This feature request is too specific for me to add. If you'd still like to see this implemented, you can commission me to make a separate mod or you can code it yourself.

A separate mod. I don't blame you for not wanting to maintain your own mod, but I also don't want to maintain code for a feature like this.

Code it my self as in, as a PR, or a separate mod? If the latter, I unfortunately don't have the time to maintain a mod of my own.