I am not sure if autocomplete helps in this case as the normal behavior of a select from list widget is that the list is rendered fully filled first, then, as you type in a text, the list shrinks to matching records. This is actually done in st.selectbox, it is the initial list with 200k entries that are the problem, and would be in other systems as well.
Having a key and value list would be a new feature request, which I think would be very useful. I have asked for it myself in Label and values in in selectbox. However, the workaround given is so convenient that I don’t miss not having separate key and value fields. Still, feel free to make a feature request on git.