Focusing specifically on your actual question:
Is there a better way to reserve a spot in the answers that publishing an incomplete answer and Updating it later?
No, there isn't. The way you attempted was to post saying "Working on an answer, will update this in a bit", which is posting a non-answer as an answer. That is already against the policy of "Only answers should be posted as answers".
There's no method to 'reserve' a spot in the answers. It's a "Post an answer before it's closed" race condition, there's no way to solve that.
The other proposed way you stated via a comment was to post an answer, then delete it, and later undelete it once it's actually an answer.
Thanks @ Thomas Ward: Is there a way to post, delete and resurrect an answer? If I spend the time researching and writing and testing an answer I would like to know that I can publish it
Nope, same race condition, and same problem as what you actuall yiddid - you post a non-answer, then delete it because you know it's not an answer, only to undelete later when it is an answer. This breaks the first rule since you're posting a non-answer as an answer. The only difference is you have the chance to undelete it after it is an answer, because it wasn't mod-deleted for not being an answer.
You still broke the "Don't post non-answers as answers" problem.
There is no solution to the problem of answering before a question gets closed as a duplicate or as unclear or {insert close reason here}.
There is also no way to reserve such a spot for your post - you either post your answer before it's closed and are lucky, or a post gets closed first. That race condition continues to exist, and though it's had proposals to fix it in the past they haven't gone anywhere.