Question?
Why can’t you set up a list variable straight away and be constant. .
The only way I have found a way to create a variable list is to populate from the piston set variable. With each index number. Once that is done it will save the values. You can then remove the sevariable command from the piston after first run and they will remain, and can be referenced.
Just kind of clumsy way of doing it.
Short answer is that support for lists was added to solve a specific problem so there are many, many features around lists that are needed but not yet supported.
I submitted a proposal for a new input type that would handle pre-populating lists among other things but there was not much interest. I had start to work on it to make sure it was possible and to get screenshots, but there is a good chunk left to do and it will require some feedback to confirm that people would use it or to suggest better alternatives. Feel free to comment there and revive the discussion!