As a suggestion that takes the concept of ASPD away from the source, depending on what you want to do with it, you can do it using functions like onpcloginevent and sc_itemscript.
For instance, If your server wants to promote higher ASPD when players level up, you can set an itemscript that gets applied to players depending on their levels, using the doppelganger card item script, you can adjust a passive aspd bonus using sc_itemscript depending on the variables you want to set out.
This moves changing entire formula of ASPD in Rathena and allows you to focus on individual variables that can affect it